Editing Data cleaning


== check if field value was not fulfilled  ==
=== By purpose ===
<table border="1" style="width: 100%">
  <tr style="background-color: #555555; color: #ffffff;">
    <td style="background-color: #777; color: #ffffff;">Purpose</td>
    <td style="background-color: #777; color: #ffffff; width: 350px;">Method (MySQL query syntax)</td>
    <td style="vertical-align: top;">Value1: <br />Fulfilled value what I want</td> 
    <td style="vertical-align: top;">Value2: <br />Fulfilled value NOT I want</td>
    <td style="vertical-align: top;">Value3: <br />0</td>
    <td style="vertical-align: top;">Value4: <br />NULL value</td>
    <td style="vertical-align: top;">Value5: <br />Empty or white-spaces characters</td>
  </tr>
  <tr>
    <td>values were not fulfilled or empty <br />(not contains 0)</td>
    <td>{{kbd | key=WHERE column_name IS NULL <br /> OR LENGTH(TRIM( column_name )) = 0}}</td> 
    <td></td>
    <td></td>
    <td></td>
    <td>V</td> 
    <td>V</td>
  </tr>
  <tr>
    <td>values were not fulfilled or empty<br />(contains 0)</td>
    <td></td> 
    <td></td>
    <td></td>
    <td>V</td>
    <td>V</td> 
    <td>V</td>
  </tr>
  <tr>
    <td>values were fulfilled and non-empty<br />(not contains 0)</td>
    <td></td> 
    <td>V</td>
    <td>V</td>
    <td></td>
    <td></td> 
    <td></td>
  </tr>
  <tr>
    <td>values were fulfilled and non-empty<br />(contains 0)</td>
    <td>{{kbd | key=<nowiki>WHERE LENGTH(TRIM( column_name )) > 0</nowiki>}}</td> 
    <td>V</td>
    <td>V</td>
    <td>V</td>
    <td></td> 
    <td></td>
  </tr>
  <tr>
    <td>values (1) were not fulfilled or empty values (2) NOT I want <br />(not contains 0)</td>
    <td>{{kbd | key=WHERE column_name IS NULL <br /> OR LENGTH(TRIM( column_name )) = 0 <br /> OR column_name LIKE 'values NOT I want'}}</td> 
    <td></td>
    <td>V</td>
    <td></td>
    <td>V</td> 
    <td>V</td>
  </tr>
  <tr>
    <td>values (1) were not fulfilled or empty values (2) NOT I want <br />(contains 0)</td>
    <td></td> 
    <td></td>
    <td>V</td>
    <td>V</td>
    <td>V</td> 
    <td>V</td>
  </tr>
</table>

=== by datatype ===
==== VARCHAR and allows NULL value ====
<table border="1" style="width: 100%">
  <tr style="background-color: #555555; color: #ffffff;">
    <td>data type of column</td>
    <td>possible column values</td>
    <td>method1: <br />find not fulfilled or empty values</td> 
    <td>method2: <br />find fulfilled and non-empty values</td>
    <td>method3: <br />find NULL values</td>
    <td>method4: <br />find not NULL values</td>
  </tr>
  <tr>
     <td rowspan="5">{{kbd | key = VARCHAR}} and allows {{kbd | key = NULL}}</td>
    <td>fulfilled value ex:123</td>
    <td></td> 
    <td>V</td>
    <td></td>
    <td>V</td> 
  </tr>
  <tr>
    <td>{{kbd | key = NULL}} type:null</td>
    <td>V</td> 
    <td></td>
    <td>V</td>
    <td></td> 
  </tr>
  <tr>
    <td>{{kbd | key = 'NULL'}} type:string</td>
    <td></td> 
    <td></td>
    <td></td>
    <td></td> 
  </tr>
  <tr>
    <td>0</td>
    <td></td> 
    <td>V</td>
    <td></td>
    <td>V</td> 
  </tr>
  <tr>
    <td>EMPTY ex: <nowiki>'' or space(s) '  '</nowiki></td>
    <td>V</td> 
    <td></td>
    <td></td>
    <td>V</td>
  </tr>
</table>

symbol V: means the column value will be able to find by means of the method


* method1: 
** {{kbd | key = <nowiki>SELECT * FROM `my_table` WHERE COALESCE(column_name, '') = ''</nowiki>}}<ref>[http://www.w3resource.com/mysql/comparision-functions-and-operators/coalesce-function.php MySQL COALESCE() function - w3resource]</ref>
** {{kbd | key = <nowiki>SELECT * FROM `my_table` WHERE column_name IS NULL OR LENGTH(TRIM( column_name )) = 0</nowiki>}}
**{{kbd | key = <nowiki>SELECT * FROM `my_table` WHERE column_name IS NULL OR column_name = ''</nowiki>}}<ref>[http://stackoverflow.com/questions/17832906/how-to-check-if-field-is-null-or-empty-mysql How to check if field is null or empty mysql? - Stack Overflow]</ref>
* method2: 
** {{kbd | key = <nowiki>SELECT * FROM `my_table` WHERE column_name > ''</nowiki>}}
** {{kbd | key = <nowiki>SELECT * FROM `my_table` WHERE LENGTH(TRIM( column_name )) > 0</nowiki>}}
** {{kbd | key = <nowiki>SELECT * FROM `my_table` WHERE LENGTH(TRIM( column_name )) != 0</nowiki>}}
* method3: {{kbd | key = <nowiki>SELECT * FROM `my_table` WHERE column_name IS NULL</nowiki>}}
* method4: {{kbd | key = <nowiki>SELECT * FROM `my_table` WHERE column_name IS NOT NULL</nowiki>}}

==== VARCHAR or numeric ====
<table border="1" style="width: 100%">
  <tr style="background-color: #555555; color: #ffffff;">
    <td>data type of column</td>
    <td>possible column values</td>
    <td>method5: <br />find values within the range</td> 
    <td>method6: <br />find values out of the range, empty & NULL values</td>
  </tr>
  <tr>
    <td rowspan="4">{{kbd | key = VARCHAR}} or numeric</td>
    <td>values within the range ex:  min ≤ value ≤ max</td>
    <td>V</td> 
    <td></td>
  </tr>
  <tr>
    <td>values out of range</td>
    <td></td> 
    <td>V</td>
  </tr>
  <tr>
    <td>{{kbd | key = NULL}}</td>
    <td></td> 
    <td>V</td>
  </tr>
  <tr>
    <td>EMPTY ex: <nowiki>'' or space '  '</nowiki></td>
    <td></td> 
    <td>V</td>
  </tr>
</table>

* method5: {{kbd | key = <nowiki>SELECT * FROM `my_table` WHERE column_name BETWEEN min AND max</nowiki>}}
* method6: {{kbd | key = <nowiki>SELECT * FROM `my_table` WHERE (   (COALESCE(column_name, '') = '') OR (column_name NOT BETWEEN min AND max)   )</nowiki>}}

==== datetime and allows NULL value ====
possible column values
# {{kbd | key = {{Template:Today}}}}
# {{kbd | key = 00-00-00 00:00:00}}
# {{kbd | key = NULL}}

=== is null ===
Fill 0 if the value is NA or NULL
* MySQL SQL syntax: COALESCE(): {{kbd | key = SELECT COALESCE(column_name, 0) }} or {{kbd | key = SELECT COALESCE(column_name, 'other_filled_value') }} 
** (1) Using [http://www.w3resource.com/mysql/comparision-functions-and-operators/coalesce-function.php COALESCE() function] to replace the NULL value with 0. 
** (2) The case: 0/0 = null should be handled.
* MySQL SQL syntax: combined IF() & ISNULL():{{kbd | key = SELECT IF(ISNULL((column_name), 0, column_name) }} or  {{kbd | key = SELECT IF(ISNULL((column_name), 'other_filled_value', column_name) }}
* python: [http://pandas.pydata.org/pandas-docs/dev/generated/pandas.DataFrame.fillna.html pandas.DataFrame.fillna — pandas 0.16.0 documentation] "Fill NA/NaN values using the specified method"

Find whether a variable is NULL. [http://ideone.com/tKv2gQ online demo]
* PHP [http://tw2.php.net/is_null is_null] to find type:null null NOT type:string <nowiki>'null'</nowiki> {{exclaim}}
* Google spreadsheet / Excel: 
** [https://support.google.com/drive/answer/3093348 ISERR(value)] " value - The value to be verified as an error type other than #N/A." ex: {{kbd | key = #NULL!}}
** If the cell value is exactly {{kbd | key = NULL}} not {{kbd | key = #NULL!}}, You may use  {{kbd | key = <nowiki>COUNTIF(value, "NULL")</nowiki>}} or  {{kbd | key = <nowiki>EXACT(value, "NULL")</nowiki>}}
* MySQL SQL syntax: {{kbd | key = SELECT * FROM table WHERE column IS NULL;}}<ref>[http://dev.mysql.com/doc/refman/5.0/en/working-with-null.html MySQL :: MySQL 5.0 Reference Manual :: 3.3.4.6 Working with NULL Values]</ref> 
* R: is.null(): [https://stat.ethz.ch/R-manual/R-devel/library/base/html/NULL.html R: The Null Object]
* Excel<ref>[https://errerrors.blogspot.com/2018/09/check-if-cell-value-is-null-in-excel.html 如何判斷 Excel 儲存格的欄位值是 NULL]</ref>

Find whether a variable is NOT NULL
* MySQL SQL syntax: {{kbd | key = SELECT * FROM table WHERE column IS NOT NULL;}}

Find whether a variable is NOT {{kbd | key = #N/A}}
* Excel: {{kbd | key = =NOT(ISERROR(cell_value))}}

javascript
* [http://www.w3schools.com/jsref/jsref_isnan.asp JavaScript isNaN() Function]

=== check if field value was not fulfilled: NULL, empty value ===
{{exclaim}} NOT include those data which its field value  fulfilled with default value automatically ([http://sqlfiddle.com/#!2/9b01e/3/0 demo on sqlfiddle])

# {{Gd}} quick solution: find records with {{kbd | key = NULL}} value OR empty, space value
#* MySQL solution: {{kbd | key = SELECT * FROM table_name WHERE column_name IS NULL OR LENGTH(TRIM( column_name )) = 0;}} 
# find records with {{kbd | key = NULL}} value: (note: not {{kbd | key = #NULL!}})
#* MySQL solution: {{kbd | key = SELECT * FROM table_name WHERE column_name IS NULL;}} 
#* EXCEL: {{kbd | key =<nowiki>=EXACT(A2, "NULL")</nowiki>}}
# find records with empty value: (not contains {{kbd | key = NULL}} value)
#* MySQL: {{kbd | key = SELECT * FROM table_name WHERE LENGTH(TRIM( column_name )) = 0;}} {{Exclaim}} SQL query {{kbd | key =SELECT * FROM table_name WHERE column_name IS NOT NULL}} includes empty value 
#* MS SQL Server: {{kbd | key = SELECT * FROM table_name WHERE LEN( LTRIM(RTRIM(column_name)) ) = 0;}} <ref>[http://www.1keydata.com/tw/sql/sql-trim.html SQL TRIM 函數 - 1Keydata SQL 語法教學]</ref>
#  Excel starting date: 1900/1/0 (converted time formatted value from 0), 1900/1/1 (converted time formatted value from 1), 1900/1/2 ... 
#* solution: step1: Replace the year > 100 from this year with empty value at EXCEL: {{kbd | key =<nowiki>=IF(ISERR(YEAR(A2)), "", IF(YEAR(A2)<1914, "", A2))</nowiki>}} (this formula also handle empty value and non well-formatted column value ex: 0000-12-31 ) ; step2: change the format of cell to time format
#* trivial approach : EXCEL: {{kbd | key =<nowiki>=IF(ISERR(YEAR(A2)), "", IF(YEAR(A2)-YEAR(NOW())>100, "", A2))</nowiki>}}  {{exclaim}} this formula could not handle empty value because it return 0. If I change the format of cell to time format, 0 will become 1900/1/0.
# Using PHP 
#* [http://php.net/manual/en/function.empty.php empty()] function to find 0, null, false, empty string, empty array values.
#* <code>if(empty($var) && $var !== 0 && $var !== "0"){ .. } </code> to find null, false, empty string, empty array values BUT not 0.
# check if field value was NULL & not equal to some value
#* MySQL: [http://stackoverflow.com/questions/17097890/mysql-select-on-fields-containing-null-values MySql select on fields containing null values - Stack Overflow]

=== check if field value was fulfilled ===
length of string > 0
* MySQL: {{kbd | key = SELECT * FROM table_name WHERE LENGTH(TRIM( column_name )) != 0;}} [http://sqlfiddle.com/#!9/e0e5f/1/0 demo][http://ideone.com/rt6D2A]

column value is not {{kbd | key = null}} or {{kbd | key = 0}}
* Excel: {{kbd | key = COUNTIFS(criteria_range1, "<>NULL", criteria_range1, "<>0")}}<ref>[https://www.ablebits.com/office-addins-blog/2014/07/10/excel-countifs-multiple-criteria/ Excel COUNTIFS and COUNTIF with multiple criteria – examples of usage]</ref>

find if number or cell value is positive integer
* EXCEL: {{kbd | key =<nowiki>=IFERROR(IF(AND(INT( value )= value, value>0), TRUE, FALSE), FALSE)</nowiki>}}<ref>[http://www.ozgrid.com/forum/showthread.php?t=24464 Check if number is an Integer]</ref> [https://docs.google.com/spreadsheets/d/1JlY9RsFDO-I9BuFa_Oegnrq5-V5vJwt4jVYUZBlyFLM/edit?usp=sharing online demo]

check numeric range
* MySQL: {{kbd | key = SELECT * FROM table_name WHERE column_name BETWEEN ''min_number'' AND ''max_number'';}} the value >= ''min_number'' AND value <= ''max_number''  ( ''min_number'' ≤ value ≤ ''max_number'' )

find NOT empty records means records without NULL or empty value: 
* MySQL: {{kbd | key =<nowiki>SELECT * FROM table_name WHERE LENGTH(TRIM( column_name )) != 0;</nowiki>}}
* MySQL: {{kbd | key =<nowiki>SELECT * FROM table_name WHERE column_name != '' AND column_name IS NOT NULL;</nowiki>}}

== Verify the format of field value ==
related page: [[Regular expression]]

=== Email contains @ symbol ===
* EXCEL: {{kbd | key =<nowiki>=IF(ISERR(FIND("@", A2, 1)), FALSE, TRUE)</nowiki>}} only check the field if contains @ symbol or not
** result: (1) normal condition: return TRUE; (2) exceptional condition: return '''FALSE''' if @ symbol was not found 
* EXCEL: {{kbd | key =<nowiki>=FIND("@", A2, 2)</nowiki>}} only check the field if contains @ symbol or not
** syntax: <nowiki>FIND(find_text, with_text, [start_num])</nowiki> the start_num is 2 because the position of  @ symbol should be larger than 1 (position of first char is 1)
** result: (1) normal condition: return the number larger than 1; (2) exceptional condition: return '''#VALUE!''' if @ symbol was not found 
* PHP: [http://www.w3schools.com/php/filter_validate_email.asp PHP FILTER_VALIDATE_EMAIL Filter]
** "Returns the filtered data, or '''FALSE''' if the filter fails." quoted from [http://php.net/manual/en/function.filter-var.php PHP.net]

=== Number precision in Excel ===
Number precision: 15 digits (Excel中最多的有效位數為15位)<ref>[https://support.office.com/en-nz/article/Excel-specifications-and-limits-1672b34d-7043-467e-8e27-269d656771c3 Excel specifications and limits]</ref><ref>[http://isvincent.pixnet.net/blog/post/30267208-excel-%E9%A1%AF%E7%A4%BApi%E8%BC%83%E5%A4%9A%E7%9A%84%E6%9C%89%E6%95%88%E4%BD%8D%E6%95%B8 A2]</ref>

raw data: 1234567890123456
->
* (numeric format 數值格式) 1234567890123450.00 {{exclaim}} losing precision
* (general format 通用格式) 1.23457E+15 {{exclaim}} losing precision
* (text format 文字格式) 1234567890123456

large numbers
* If the data was imported from Excel, you should notice the 15 digit precision issue.

=== Numeric ===
List of the possible abnormal values:
* All numeric values are odd or even if the data were generated by user naturally.

PHP: 
* [http://php.net/manual/en/function.is-numeric.php is_numeric]

MySQL: 
* Find the records which the value of `my_column` is numeric values entirely {{code | code = SELECT * FROM `my_table` WHERE `my_column` REGEXP '^[0-9]+$'}}<ref>[http://stackoverflow.com/questions/14343767/mysql-regexp-with-and-numbers-only regex - Mysql REGEXP with . and numbers only - Stack Overflow]</ref>
* Find the records which the value of `my_column` is '''NOT''' numeric values entirely {{code | code = SELECT * FROM `my_table` WHERE `my_column` NOT REGEXP '^[0-9]+$'}}

If the digit of number is known, the SQL syntax could be more specific
* The {{kbd | key=tax_id}} column is 8 digits only. Find the well-formatted {{kbd | key=tax_id}} records by using {{code | code = SELECT * FROM `tax_id` WHERE `tax_id` REGEXP '^[0-9]{8}$'}}

Excel & [https://www.google.com/sheets/about/ Google Sheets]: 
* Using [http://www.techonthenet.com/excel/formulas/isnumber.php ISNUMBER Function]: {{code | code = <nowiki>=INT(ISNUMBER(A1))</nowiki>}}
** Return 1 if the cell value is (1) Numbers (2) Numbers in [https://en.wikipedia.org/wiki/Scientific_notation scientific (exponential) notation] e.g. {{code | code = <nowiki>1.36184E+14</nowiki>}} (3) Decimal numbers e.g. {{code | code = <nowiki>3.141592654</nowiki>}} (4) Negative numbers
** Return 0 if the cell value is (1) Text (2) Numbers that are stored as text e.g. {{code | code = <nowiki>="5"</nowiki>}}

*  Google Sheets only: Using [https://support.google.com/docs/answer/3098292?hl=zh-Hant REGEXMATCH], [https://support.google.com/docs/answer/3094140?hl=zh-Hant TRIM] & [https://support.google.com/docs/answer/3093592?hl=zh-Hant CONCAT]<ref>[https://errerrors.blogspot.com/2015/08/google.html GOOGLE 試算表: 數字轉成文字]</ref> functions: {{code | code = <nowiki>=IF(REGEXMATCH(CONCAT("", TRIM(A1)), "^\d+$"), 1, 0)</nowiki>}}
** Return 1 if the cell value is (1) Numbers (2) Numbers that are stored as text e.g. {{code | code = <nowiki>="5"</nowiki>}}
** Return 0 if the cell value is (1) Text (2) Numbers in scientific (exponential) notation e.g. {{code | code = <nowiki>1.23E+16</nowiki>}} (3) Decimal numbers e.g. {{code | code = <nowiki>3.141592654</nowiki>}} (4) Negative numbers

<pre>
test data:
3.141592654
1.36184E+14
20740199601
346183773390240
="5"
</pre>

=== Time data: the value should be YYYY/MM/DD format ===

<pre>
SELECT `my_date_column`, UNIX_TIMESTAMP(STR_TO_DATE(`my_date_column`, '%Y/%m/%d')) 
FROM `my_table`
WHERE
UNIX_TIMESTAMP(STR_TO_DATE(`my_date_column`, '%Y/%m/%d')) IS NULL;

</pre>

=== Time data: Data was generated in 10 years ===
Definition of abnormal values of the time data ([http://en.wikipedia.org/wiki/Time_series time series]) if they
* were generated 10 years before or
* newer than today

List of the possible abnormal values:
* {{code | code = 0001-01 00:00:00}} occurred in MySQL {{code | code = datetime}} type
* {{code | code = 1900/1/0}} (converted time formatted value from 0), {{code | code = 1900/1/1}} (converted time formatted value from 1), {{code | code = 1900/1/2}} ... occurred in MS Excel
* future data: the date after today

Find the normal values: 
* MySQL: Assume the data was generated in recent 10 years & not newer than today
** {{code | code = SELECT * FROM `my_table` WHERE ( `my_time_column` >=  CURDATE() - INTERVAL 10 YEAR )   AND   ( `my_time_column` < CURDATE() + 1);}} 
*** {{exclaim}} NOT {{code | code =   `my_time_column` < CURDATE() }}。 ex: {{code | code = CURDATE() }} is {{CURRENTYEAR}}-{{CURRENTMONTH}}-{{CURRENTDAY2}}. Which is the same with {{CURRENTYEAR}}-{{CURRENTMONTH}}-{{CURRENTDAY2}} 00:00:00
** {{code | code = SELECT * FROM `my_table` WHERE ( YEAR( CURDATE() ) - YEAR( `my_time_column`) <= 10 )   AND   ( `my_time_column` < CURDATE() + 1); }}
* MySQL: Assume the data was generated in recent 10 years & not newer than current timestamp. More precision to second compared with the above approach. 
** {{code | code = SELECT * FROM `my_table` WHERE ( `my_time_column` >=  CURDATE() - INTERVAL 10 YEAR )   AND   ( `my_time_column` <= CURRENT_TIMESTAMP);}} 
*** You need to check the {{code | code = SELECT CURRENT_TIMESTAMP);}} if correct or not before you delete the abnormal data (timezone issue)

Check if the date valid
* Excel: [https://support.office.com/en-us/article/WEEKDAY-function-60e44483-2ed1-439f-8bd0-e404c190949a?ui=en-US&rs=en-US&ad=US WEEKDAY function] <ref>[http://www.mrexcel.com/forum/excel-questions/322203-check-if-valid-date.html Check if a valid date?]</ref>
** legal results: {{kbd | key=1}} (Sunday) ~ {{kbd | key=7}} (Saturday)
** illegal result: {{kbd | key= <nowiki>#VALUE!</nowiki>}}
* PHP: [http://stackoverflow.com/questions/19271381/correctly-determine-if-date-string-is-a-valid-date-in-that-format php - Correctly determine if date string is a valid date in that format - Stack Overflow]

=== Time data: Human birth year (age) data ===
Based on the existing record, the longest-living person who lived to 122<ref>[https://en.wikipedia.org/wiki/Maximum_life_span Maximum life span - Wikipedia]</ref>. 

MySQL query is as follows<ref>[https://stackoverflow.com/questions/5773405/calculate-age-in-mysql-innodb sql - Calculate Age in MySQL (InnoDb) - Stack Overflow]</ref> where the column {{kbd | key=<nowiki>`birthday`</nowiki>}} is {{kbd | key=<nowiki>date</nowiki>}} type.
<pre>
WHERE TIMESTAMPDIFF(YEAR, `birthday`, CURDATE()) <= 122
</pre>

Using [https://www.w3resource.com/mysql/date-and-time-functions/mysql-unix_timestamp-function.php UNIX_TIMESTAMP() function] to check the abnormality of birthday data is not appropriate. Because the birthdays which are earlier {{kbd | key=<nowiki>1970-01-01 00:00:00 UTC</nowiki>}} will all become zero.


=== String contains special characters ===
* [[Byte order mark]] (BOM)
* [[Return symbol]]
* [http://www.fileformat.info/info/unicode/char/a0/index.htm Unicode Character 'NO-BREAK SPACE' (U+00A0)]

== Duplicate data ==
=== Find duplicate data ===
==== EXCEL ====
===== Finding duplicate rows that differ in one column =====
* one column data: [http://www.extendoffice.com/documents/excel/1499-count-duplicate-values-in-column.html How to count duplicate values in a column in Excel?] Using {{kbd | key = COUNTIF(range, criteria)}} {{access | date = 2015-08-25}} or using '''Pivot Tables'''(樞紐分析表)  to find the occurrence of value >= 2

===== Finding duplicate rows that differ in multiple columns =====
* two or multiple columns data: (approach 1) [https://support.microsoft.com/en-us/kb/213367 How to compare data in two columns to find duplicates in Excel] {{access | date = 2015-06-16}} {{exclaim}} It may costs too much time (larger than one hour) if the number of records exceeds 1,000,000 (approach 2) Using [https://support.office.com/en-us/article/concat-function-9b1a9a3f-94ff-41af-9736-694cbd6b4ca2 CONCAT function] to concatenate two or multiple columns data. And then use {{kbd | key = COUNTIF(range, criteria)}}.

==== Cygwin ====
* [https://www.computerhope.com/unix/uuniq.htm uniq command] on Cygwin of {{Win}} or {{Linux}}: {{kbd | key=<nowiki>uniq -d <file.txt> > <duplicated_items.txt></nowiki>}}<ref>[https://unix.stackexchange.com/questions/52534/how-to-print-only-the-duplicate-values-from-a-text-file shell - How to print only the duplicate values from a text file? - Unix & Linux Stack Exchange]</ref>

==== MySQL ====
===== Finding duplicate rows that differ in one column =====
Find the duplicated data for one column<ref>[http://stackoverflow.com/questions/688549/finding-duplicate-values-in-mysql?rq=1 Finding duplicate values in MySQL - Stack Overflow]</ref>
<pre>
-- Generate test data.
CREATE TABLE `table_name` (
  `id` int(11) NOT NULL,
  `content` varchar(5) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO `table_name` (`id`, `content`) VALUES
(1, 'apple'),
(2, 'lemon'),
(3, 'apple');

ALTER TABLE `table_name`
  ADD PRIMARY KEY (`id`);

-- Find duplicated data
SELECT `content`, COUNT(*) count 
FROM `table_name` 
GROUP BY `content` 
HAVING count > 1;

SELECT tmp.* FROM 
( 
  SELECT `content`, count(*) count FROM `table_name` GROUP BY `content` 
) tmp 
WHERE tmp.count >1;
</pre>

===== Finding duplicate rows that differ in multiple columns =====
Using {{kbd | key =CONCAT}} for multiple columns ex: column_1, column_2 
<pre>
SELECT count(*) count, CONCAT(  `column_1`, `column_2`  ) 'key'
	FROM `table_name`
	GROUP BY CONCAT(  `column_1`, `column_2`  )
HAVING count > 1;
</pre>

or
<pre>
SELECT tmp.key FROM
(
	SELECT count(*) count, CONCAT(  `column_1`, `column_2`  ) 'key'
	FROM `table_name`
	GROUP BY CONCAT(  `column_1`, `column_2`  )
) tmp
WHERE tmp.count >=2
</pre>

===== other cases =====
For counting purpose: find the count of repeated id (type: int) between table_a and table_b
<pre>
SELECT count(DISTINCT(id)) FROM table_a WHERE id IN
(
   SELECT DISTINCT(id) FROM table_b
)
</pre>

==== Google Spreadsheet ====

* [https://www.ablebits.com/google-sheets-add-ons/remove-duplicates/index.php Remove duplicates in Google Sheets] 30 days free {{access | date = 2019-02-26}}
* [https://chrome.google.com/webstore/detail/power-tools/dofhceeoedodcaheeoacmadcpegkjobi Power Tools] for Google Spreadsheet {{access | date = 2019-02-26}}
** Menu: Data -> Remove duplicates

=== Deduplicate ===
* EXCEL: Data Tools -> Remove Duplicates: [https://support.office.com/en-us/article/Filter-for-unique-values-or-remove-duplicate-values-d6549cf0-357a-4acf-9df5-ca507915b704 Filter for unique values or remove duplicate values] {{access | date = 2015-10-20}}

* PHP: [http://php.net/manual/en/function.array-unique.php PHP: array_unique], [http://php.net/manual/en/function.array-intersect.php PHP: array_intersect]

* MySQL: select deduplicated records
** [http://www.mysqltutorial.org/mysql-distinct.aspx MySQL DISTINCT - Eliminate Duplicate Rows in a Result Set]. Using {{kbd | key =GROUP_CONCAT}} to handle the multiple columns<ref>[http://stackoverflow.com/questions/12188027/mysql-select-distinct-multiple-columns sql - MySQL SELECT DISTINCT multiple columns - Stack Overflow]</ref>
** [http://www.w3schools.com/sql/sql_unique.asp SQL UNIQUE Constraint] "Note that you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table." Quoted from w3schools webpage.
** "{{kbd | key = UNION}} removes duplicates, whereas {{kbd | key = UNION ALL}} does not." source: [http://stackoverflow.com/questions/49925/what-is-the-difference-between-union-and-union-all sql - What is the difference between UNION and UNION ALL? - Stack Overflow]
* MySQL: delete duplicated records
** [http://stackoverflow.com/questions/4685173/delete-all-duplicate-rows-except-for-one-in-mysql sql - Delete all Duplicate Rows except for One in MySQL? - Stack Overflow]

* [http://www.gnu.org/software/coreutils/manual/html_node/sort-invocation.html GNU Coreutils: sort invocation] OS: {{Linux}}, cygwin of {{Win}}. More details on [[Alternative_Linux_commands#Merge_multiple_plain_text_files | Merge multiple plain text files]].
** To remove duplicate lines: 
*** {{kbd | key=<nowiki>sort -us -o <output_unique.file> <input.file></nowiki>}} in a large text file (GB)<ref>[http://unix.stackexchange.com/questions/19641/how-to-remove-duplicate-lines-in-a-large-multi-gb-textfile linux - How to remove duplicate lines in a large multi-GB textfile? - Unix & Linux Stack Exchange]</ref> 
*** {{kbd | key=<nowiki>cat <input.file> | grep <pattern> | sort | uniq</nowiki>}} Processes text line by line and prints the '''unique''' lines which match a specified pattern. Equal to these steps: (1) {{kbd | key=<nowiki>cat <input.file> | grep <pattern> > <tmp.file></nowiki>}} (2) {{kbd | key=<nowiki>sort <tmp.file> | uniq</nowiki>}}
** Ignore first n line(s) & remove duplicate lines<ref>[https://stackoverflow.com/questions/14562423/is-there-a-way-to-ignore-header-lines-in-a-unix-sort sorting - Is there a way to ignore header lines in a UNIX sort? - Stack Overflow]</ref><ref>[http://linux.vbird.org/linux_basic/0320bash.php#redirect_com 命令執行的判斷依據： ; , &&, ||]</ref><ref>[https://www.computerhope.com/unix/utail.htm Linux tail command help and examples]</ref>
*** (1) ignore first one line: {{kbd | key=<nowiki>(head -n 1 <file> && tail -n +2 <file> | sort -us) > newfile</nowiki>}}  
*** (2) ignore first two lines: {{kbd | key=<nowiki>(head -n 2 <file> && tail -n +3 <file> | sort -us) > newfile</nowiki>}}

* Google spreadsheet add-on: [https://www.ablebits.com/google-sheets-add-ons/remove-duplicates/howto.php Remove Duplicates for Google Sheets help]

=== Counting number of duplicate occurrence ===
MySQL: find the number of duplicate occurrence between list_a & list_b which using the same primary key: column name {{kbd | key = id}}
* {{kbd | key = SELECT count(DISTINCT(`id`)) FROM `list_a` WHERE `id` IN (SELECT DISTINCT(`id`) FROM `list_b`) ; }}

Excel: 
* [http://superuser.com/questions/307837/how-to-count-number-of-repeat-occurrences microsoft excel - How to count number of repeat occurrences - Super User] {{exclaim}} long number issue: [https://superuser.com/questions/783840/countif-incorrectly-matches-long-number microsoft excel - Countif incorrectly matches long number - Super User]

=== Other ===
* symbol e.g. data-mining or data_mining

== Counting ==

[[Count occurrences of a word in string]]

== Outlier / Anomaly detection ==
Anomaly detection of numeric data
* Median
* Range Checks
* All values is event
* The values are the same even the column is totally different

Anomaly detection of categorical data (qualitative variable)
* Normal distribution e.g. The interest of audiences should be very different NOT coherent

Anomaly detection for time series data
* Trend
* Dramatically Increase or decrease of rows count for each time period

More on: [https://en.wikipedia.org/wiki/Outlier#Identifying_outliers Outlier - Wikipedia]

== unique number of data values ==
If the data values were generated by different users, the unique number of data values should be larger than ____

== Data handling ==
=== Remove first, last or certain characters from text ===
* Excel: using {{kbd | key=RIGHT}}<ref>[http://office.microsoft.com/zh-tw/excel-help/HP010342854.aspx RIGHT、RIGHTB 函數 - Excel - Office.com]</ref> + {{kbd | key=LEN}}<ref>[http://office.microsoft.com/zh-tw/excel-help/HP010342650.aspx LEN、LENB 函數 - Excel - Office.com]</ref> functions <ref>[http://www.extendoffice.com/documents/excel/560-excel-remove-character-from-string.html How to remove first, last or certain characters from text in Excel?]</ref>
* Excel: if the length of text was fixed after removed, you may try to use {{kbd | key=REPLACE}}<ref>[http://office.microsoft.com/zh-tw/excel-help/HP010342844.aspx REPLACE、REPLACEB 函數 - Excel - Office.com]</ref> + {{kbd | key=LEN}} functions ([http://bit.ly/1i2tsgk demo])

=== Remove leading and trailing spaces from text ===
* PHP: [http://php.net/manual/en/function.trim.php trim function]
* Excel: [https://exceljet.net/formula/remove-leading-and-trailing-spaces-from-text Excel formula: Remove leading and trailing spaces from text | Exceljet]
* MySQL: Using [http://www.w3resource.com/mysql/string-functions/mysql-trim-function.php TRIM() function] & [http://www.w3resource.com/mysql/string-functions/mysql-length-function.php LENGTH() function ]
<pre>
UPDATE `table` 
SET `column` = TRIM( `column` ) 
WHERE LENGTH(TRIM( `column` )) != LENGTH( `column` );
</pre>

=== Remove other string look like whitespace ===
[https://en.wikipedia.org/wiki/Whitespace_character Whitespace character]
# [http://www.fileformat.info/info/unicode/char/3000/index.htm IDEOGRAPHIC SPACE] (全形空白、全型空白, U+3000)<ref>[https://www.ptt.cc/bbs/PHP/M.1473522044.A.4FC.html Re: 請益 mysql空百=？ - 看板 PHP - 批踢踢實業坊]</ref>:
#* diaplay: <pre><nowiki><?php $string = "111" . json_decode('"\u3000"') . "222"; echo $string;?></nowiki></pre>
#* replace with space: <pre><nowiki><?php echo str_replace(json_decode('"\u3000"'), " ", $string);?></nowiki></pre>
# ASCII Vertical Tab {{kbd | key=<nowiki>\v</nowiki>}}
# ASCII Horizontal Tab (TAB) {{kbd | key=<nowiki>\t</nowiki>}}
# ASCII Backspace {{kbd | key=<nowiki>\b</nowiki>}}
# [https://en.wikipedia.org/wiki/Non-breaking_space Non-breaking space] ({{kbd | key=<nowiki>nbsp;</nowiki>}}) Replace Non-breaking space with one whitespace using PHP: {{kbd | key=<nowiki>$result = str_replace("\xc2\xa0", ' ', $original_string);</nowiki>}}<ref>[https://stackoverflow.com/questions/40724543/how-to-replace-decoded-non-breakable-space-nbsp php - How to replace decoded Non-breakable space (nbsp) - Stack Overflow]</ref>

Sentence spacing
# [https://en.wikipedia.org/wiki/Sentence_spacing_in_digital_media Sentence spacing in digital media - Wikipedia] e.g. {{kbd | key=<nowiki>&Nbsp; &Ensp; &Emsp;</nowiki>}}

How to display the Non-breaking space In PHP?
<pre>
$input = '12345678' . hex2bin('c2a0');
echo $input . PHP_EOL;
## Result of above script: '12345678 ' (one whitespace at the end)

echo bin2hex($input) . PHP_EOL;
## Result of above script: 3132333435363738c2a0

echo bin2hex('12345678') . PHP_EOL;
## Result of above script: 3132333435363738 (You mat notice the difference of script result is C2A0)

</pre>

How to display the Non-breaking space In MySQL?
<pre>
SELECT CONCAT('12345678', UNHEX('C2A0'))
-- Result of above query: '12345678 ' (one whitespace at the end)

SELECT HEX(CONCAT('12345678', UNHEX('C2A0')))
-- Result of above query: 3132333435363738C2A0

SELECT HEX('12345678')
-- Result of above query: 3132333435363738 (You mat notice the difference of query result is C2A0)

SELECT LENGTH('12345678')
-- Result of above query: 8

SELECT LENGTH(CONCAT('12345678', UNHEX('C2A0')))
-- Result of above query: 10
</pre>

== Remove control character ==
[https://en.wikipedia.org/wiki/Control_character Control character - Wikipedia] Using PHP to clean control character:
<pre>
$input = 'some string may contains control characters';
$replacement = '';
$result = preg_replace('/[\x00-\x1F]/', $replacement, $input);
</pre>

=== Fix garbled message text ===
[[Fix garbled message text]]

== Further reading ==
* [https://g0v.gitbooks.io/data-design/content/book/ch08-data-cleaning.html Data Cleaning | DATA + DESIGN 資料＋設計]
* Data modeling: [[Data type]]
* [http://www.icpsr.umich.edu/icpsrweb/NAHDAP/support/faqs/2006/01/what-is-codebook What is a codebook?]
* [https://www.facebook.com/groups/1210634969026548/permalink/1540468379376537/ 台灣R軟體Club] 「由於真實社會的資料千奇百怪，遺失值、空字串、空格、甚至是假的 "NA" (NA兩邊多了雙引號)、假的"NULL" (NULL兩邊多了雙引號) 。。。，其中一個顯著的差異是，社會實務應用的 R 程式中，通常會多出一堆檢查機制。 ... ...」{{access | date = 2017-12-18}}
* [https://docs.google.com/spreadsheets/d/1UyumfruGjps7xqd4aSB-NKfmu54NIscI2OV9Uht8F7M/edit?usp=sharing  判斷 Excel 欄位值是否錯誤] 除了常見的 Excel 錯誤，如何判斷來自其他資料來源的可能錯誤欄位值
* [[Web user behavior]]
* [https://medium.com/bryanyang0528/%E8%B3%87%E6%96%99%E5%93%81%E8%B3%AA%E5%88%9D%E6%8E%A2-data-quality-b765eb56a7c2?fbclid=IwAR3NBb2BtFm9O3FeY7JgQ5HLE5VG5nFe3m5Zx8zNW9XkvOUPlqV9hXmaoXI 資料品質初探(Data Quality) – 亂點技能的跨界人生 – Medium] {{access | date = 2018-01-13}}

== references ==
<references/>

[[Category:Spreadsheet]] [[Category:Excel]]
[[Category:Data_hygiene]]
[[Category:Data Science]]
[[Category:MySQL]]
[[Category:Text file processing]]