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Summary of MySQL common judgment functions!! Have you used it

2022-07-04 07:01:00 【Chen haha Ben ha】

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Speaking of if else You must be no stranger , This kind of judgment function is common in various programming languages , But I'm writing SQL In the sentence , Maybe you rarely use , I haven't even played much yet .
stay MySQL The function based on the judgment of condition is also called “ Control flow function ”, be used for mysql Logical judgment in statements . Let's take a look at MySQL What are the common control flow functions in , And the use scenarios of control flow function ？

MySQL Summary of common judgment functions ！！ See if you've used it _ Control flow function

Catalog

One 、 function ：CASE WHEN ... THEN ... ELSE ... END

1、 Used in update conditions of update statements
2、 Used in the return value of a query statement
3、 Used in group query statements

Two 、 function ：IF(expr,if_true_expr,if_false_expr)
3、 ... and 、 function ：IFNULL(expr1,expr2)
attach 、 A picture with a story （ Nine ）

One 、 function ：CASE WHEN … THEN … ELSE … END

stay SQL In the sentence ,"CASE WHEN … THEN … ELSE … END" Is a more common sentence used to judge , It is suitable for adding, deleting, modifying and checking all kinds of sentences , The formula is as follows ：

       
        CASE expression
        
WHEN if_true_expr THEN return_value1
        
WHEN if_true_expr THEN return_value2
        
WHEN if_true_expr THEN return_value3
        
……
        
ELSE default_return_value
        
END
       
       
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1、 Used in update conditions of update statements

Give me a situation 1： Women's day big feedback ,2020 New users registered in , All adult female accounts send 10 Yuan a red envelope , Other users send 5 Yuan a red envelope , Auto recharge .

The sample statements are as follows ：

       
        --  Red envelope sentence 
        
UPDATE users_info u 
        
    SET u.balance = CASE WHEN u.sex =' Woman ' and u.age > 18 THEN u.balance + 10 
        
                         ELSE u.balance + 5 end 
        
                         WHERE u.create_time >= '2020-01-01'
       
       
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Something to watch out for ,Case Function returns only the first value that matches the condition , The rest Case when Parts will be automatically ignored

2、 Used in the return value of a query statement

Give me a situation 2： There is a student's college entrance examination score table , You need to list the levels ,650 A key university with a score above ,600-650 It's a book ,500-600 There are two books ,400-500 It's three books ,400 The following College ;

The original test data are as follows ：

       
        mysql> select * from student_score;
        
+-----+-----------+-------------+------+
        
| SID | S_NAME    | TOTAL_SCORE | RANK |
        
+-----+-----------+-------------+------+
        
|   1 |  Chen ha ha     |         385 | 1760 |
        
|   2 |  Hu Yapeng     |         491 | 1170 |
        
|   3 |  Liu Xiaoli     |         508 | 1000 |
        
|   5 |  Xu Linan     |         599 |  701 |
        
|   6 |  Gu Hao       |         601 |  664 |
        
|   7 |  Chen Zi Ning     |         680 |    9 |
        
|  14 |  Zhu Zhipeng     |         335 | 1810 |
        
|  19 |  Leon       |         550 |  766 |
        
+-----+-----------+-------------+------+
        
8 rows in set (0.00 sec)
       
       
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Query statement ：

       
        SELECT *,case when total_score >= 650  THEN ' Key universities ' 
        
              when total_score >= 600 and total_score <650 THEN ' a copy '
        
              when total_score >= 500 and total_score <600 THEN ' Two copies '
        
              when total_score >= 400 and total_score <500 THEN ' Three copies '        
        
              else ' junior college ' end as status_student 
        
              from student_score;
       
       
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        mysql> SELECT *,case when total_score >= 650  THEN ' Key universities ' 
        
    ->               when total_score >= 600 and total_score <650 THEN ' a copy '
        
    ->               when total_score >= 500 and total_score <600 THEN ' Two copies '
        
    ->               when total_score >= 400 and total_score <500 THEN ' Three copies '        
        
    ->               else ' junior college ' end as status_student 
        
    ->               from student_score;
        
+-----+-----------+-------------+------+----------------+
        
| SID | S_NAME    | TOTAL_SCORE | RANK | status_student |
        
+-----+-----------+-------------+------+----------------+
        
|   1 |  Chen ha ha     |         385 | 1760 |  junior college            |
        
|   2 |  Hu Yapeng     |         491 | 1170 |  Three copies            |
        
|   3 |  Liu Xiaoli     |         508 | 1000 |  Two copies            |
        
|   5 |  Xu Linan     |         599 |  701 |  Two copies            |
        
|   6 |  Gu Hao       |         601 |  664 |  a copy            |
        
|   7 |  Chen Zi Ning     |         680 |    9 |  Key universities        |
        
|  14 |  Zhu Zhipeng     |         335 | 1810 |  junior college            |
        
|  19 |  Leon       |         550 |  766 |  Two copies            |
        
+-----+-----------+-------------+------+----------------+
        
8 rows in set (0.00 sec)
       
       
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3、 Used in group query statements

Give me a situation 3： Users include all provinces and cities in China , Statistics should be made by province , Shandong Province 、 The number of users in Guangzhou and other provinces and cities ;（ This is for testing use , In fact, there should be a column of attribution province or another attribution surface in the reasoning table .）

The data are as follows ：

       
        mysql> select * from users_area;
        
+----+--------------+-------------+
        
| id | city         | users_count |
        
+----+--------------+-------------+
        
|  1 |  Beijing          |         650 |
        
|  2 |  Shanghai          |         500 |
        
|  3 |  jinan          |         300 |
        
|  4 |  Qingdao          |         100 |
        
|  5 |  Guangzhou          |         350 |
        
|  6 |  Shenzhen          |         400 |
        
|  7 |  Zaozhuang          |         120 |
        
|  8 |  urumqi      |          80 |
        
+----+--------------+-------------+
        
8 rows in set (0.00 sec)
       
       
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Group query SQL：

       
        SELECT
        
    SUM(c.users_count) AS ' Number of users ',
        
    CASE c.city
        
        WHEN ' jinan '   THEN ' Shandong Province '
        
        WHEN ' Qingdao '   THEN ' Shandong Province '
        
        WHEN ' Zaozhuang '   THEN ' Shandong Province '
        
        WHEN ' Guangzhou '   THEN ' Guangdong province, '
        
        WHEN ' Shenzhen '   THEN ' Guangdong province, '
        
 ELSE ' other ' END AS ' Belong to the province '
        
FROM
        
    users_area c
        
GROUP BY CASE c.city
        
        WHEN ' jinan '   THEN ' Shandong Province '
        
        WHEN ' Qingdao '   THEN ' Shandong Province '
        
        WHEN ' Zaozhuang '   THEN ' Shandong Province '
        
        WHEN ' Guangzhou '   THEN ' Guangdong province, '
        
        WHEN ' Shenzhen '   THEN ' Guangdong province, '
        
 ELSE ' other ' END;
       
       
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Query results ：

       
        mysql> SELECT
        
    ->     SUM(c.users_count) AS ' Number of users ',
        
    ->     CASE c.city
        
    ->         WHEN ' jinan '   THEN ' Shandong Province '
        
    ->         WHEN ' Qingdao '   THEN ' Shandong Province '
        
    ->         WHEN ' Zaozhuang '   THEN ' Shandong Province '
        
    ->         WHEN ' Guangzhou '   THEN ' Guangdong province, '
        
    ->         WHEN ' Shenzhen '   THEN ' Guangdong province, '
        
    ->  ELSE ' other ' END AS ' Belong to the province '
        
    -> FROM
        
    ->     users_area c
        
    -> GROUP BY CASE c.city
        
    ->         WHEN ' jinan '   THEN ' Shandong Province '
        
    ->         WHEN ' Qingdao '   THEN ' Shandong Province '
        
    ->         WHEN ' Zaozhuang '   THEN ' Shandong Province '
        
    ->         WHEN ' Guangzhou '   THEN ' Guangdong province, '
        
    ->         WHEN ' Shenzhen '   THEN ' Guangdong province, '
        
    ->  ELSE ' other ' END;
        
+--------------+-----------+
        
|  Number of users      |  Belong to the province     |
        
+--------------+-----------+
        
|         1230 |  other       |
        
|          520 |  Shandong Province     |
        
|          750 |  Guangdong province,     |
        
+--------------+-----------+
        
3 rows in set (0.00 sec)
       
       
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Two 、 function ：IF(expr,if_true_expr,if_false_expr)

stay mysql in if() The usage of the function is similar to java Binocular expression in , The specific syntax is as follows ：

IF(expr,if_true_expr,if_false_expr), If expr The value of is true, Then return to if_true_expr Value , If expr The value of is false, Then return to if_false_expr Value .

Use scenarios 1：IF Function is usually used for columns where real data is replaced ; Such as gender , We usually use tinyint Storage , male = 1, Woman = 2; If the query needs to be converted into characters , This scenario applies to IF function .

The original data ：

       
        mysql> select * from student;
        
+----+-----------+-----+---------+-----------+
        
| ID | NAME      | SEX | GRADE   | HOBBY     |
        
+----+-----------+-----+---------+-----------+
        
|  1 |  Chen ha ha     | 1   | 9 grade    |  surf the internet       |
        
|  2 |  Hu Yapeng     | 1   | 9 grade    |  food       |
        
|  3 |  Liu Xiaoli     | 2   | 9 grade    |  Jinxiche     |
        
|  5 |  Xu Linan     | 2   | 9 grade    |  read       |
        
|  6 |  Gu Hao       | 1   | 9 grade    |  Basketball       |
        
|  7 |  Chen Zi Ning     | 2   | 9 grade    |  See a movie     |
        
| 14 |  Zhu Zhipeng     | 1   | 9 grade    |  Read novels     |
        
| 15 |  Jia Xu       | 1   | 9 grade    |  Brag makes you     |
        
| 19 |  Leon       | 1   | 9 grade    |  Watch the movie     |
        
+----+-----------+-----+---------+-----------+
        
9 rows in set (0.00 sec)
       
       
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Handle sex Fields are displayed in character format ;

       
        mysql> SELECT `NAME`,IF(sex = 1,' male ',' Woman ') FROM student;
        
+-----------+-------------------------+
        
| NAME      | IF(sex = 1,' male ',' Woman ')   |
        
+-----------+-------------------------+
        
|  Chen ha ha     |  male                       |
        
|  Hu Yapeng     |  male                       |
        
|  Liu Xiaoli     |  Woman                       |
        
|  Xu Linan     |  Woman                       |
        
|  Gu Hao       |  male                       |
        
|  Chen Zi Ning     |  Woman                       |
        
|  Zhu Zhipeng     |  male                       |
        
|  Jia Xu       |  male                       |
        
|  Leon       |  male                       |
        
+-----------+-------------------------+
        
9 rows in set (0.00 sec)
       
       
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If you will (1,2) Format data changed to (‘ male ’,‘ Woman ’) It can also be done through IF Function modification ( Remember to change the column type first ),SQL as follows ：

       
        mysql> UPDATE student set sex = IF(sex = 1,' male ',' Woman ');
        
Query OK, 9 rows affected (0.06 sec)
        
Rows matched: 9  Changed: 9  Warnings: 0
       
       
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Modified data ：

       
        mysql> select * from student;
        
+----+-----------+-----+---------+-----------+
        
| ID | NAME      | SEX | GRADE   | HOBBY     |
        
+----+-----------+-----+---------+-----------+
        
|  1 |  Chen ha ha     |  male   | 9 grade    |  surf the internet       |
        
|  2 |  Hu Yapeng     |  male   | 9 grade    |  food       |
        
|  3 |  Liu Xiaoli     |  Woman   | 9 grade    |  Jinxiche     |
        
|  5 |  Xu Linan     |  Woman   | 9 grade    |  read       |
        
|  6 |  Gu Hao       |  male   | 9 grade    |  Basketball       |
        
|  7 |  Chen Zi Ning     |  Woman   | 9 grade    |  See a movie     |
        
| 14 |  Zhu Zhipeng     |  male   | 9 grade    |  Read novels     |
        
| 15 |  Jia Xu       |  male   | 9 grade    |  Brag makes you     |
        
| 19 |  Leon       |  male   | 9 grade    |  Watch the movie     |
        
+----+-----------+-----+---------+-----------+
        
9 rows in set (0.00 sec)
       
       
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Use scenarios 2： Use the class chart above , Look up the total number of boys and girls ;SQL as follows ：
（sex=' male ’ Return 1, And then use SUM Add up to get the number of boys , Girls are the same .）

       
        SELECT SUM(IF(sex = ' male ',1,0)) as boyNum, SUM(IF(sex = ' Woman ',1,0)) as girlNum from student;
       
       
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        mysql> SELECT SUM(IF(sex = ' male ',1,0)) as boyNum,SUM(IF(sex = ' Woman ',1,0)) as girlNum from student;
        
+--------+---------+
        
| boyNum | girlNum |
        
+--------+---------+
        
|      6 |       3 |
        
+--------+---------+
        
1 row in set (0.00 sec)
       
       
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3、 ... and 、 function ：IFNULL(expr1,expr2)

IFNULL The function is MySQL One of the control flow functions , It has two parameters , Two parameters can be real values or expressions , If expr1 No NULL, Then return the first parameter （expr1）. otherwise ,IFNULL The function returns the second argument .

Raw data ：

       
        mysql> select * from student;
        
+----+-----------+------+---------+-----------+
        
| ID | NAME      | SEX  | GRADE   | HOBBY     |
        
+----+-----------+------+---------+-----------+
        
|  1 |  Chen ha ha     |  male    | 9 grade    |  surf the internet       |
        
|  2 |  Hu Yapeng     |  male    | 9 grade    |  food       |
        
|  3 |  Liu Xiaoli     |  Woman    | 9 grade    |  Jinxiche     |
        
|  5 |  Xu Linan     |  Woman    | 9 grade    |  read       |
        
|  6 |  Gu Hao       |  male    | 9 grade    |  Basketball       |
        
|  7 |  Chen Zi Ning     |  Woman    | 9 grade    |  See a movie     |
        
| 14 |  Zhu Zhipeng     | NULL | 9 grade    |  Read novels     |
        
| 19 |  Leon       | NULL | 9 grade    |  Watch the movie     |
        
+----+-----------+------+---------+-----------+
        
8 rows in set (0.00 sec)
       
       
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take SEX by NULL The data of the project is shown as ：‘ Unknown ’：

       
        mysql> SELECT `NAME`,IFNULL(sex,' Unknown ') from student;
        
+-----------+----------------------+
        
| NAME      | IFNULL(sex,' Unknown ')   |
        
+-----------+----------------------+
        
|  Chen ha ha     |  male                    |
        
|  Hu Yapeng     |  male                    |
        
|  Liu Xiaoli     |  Woman                    |
        
|  Xu Linan     |  Woman                    |
        
|  Gu Hao       |  male                    |
        
|  Chen Zi Ning     |  Woman                    |
        
|  Zhu Zhipeng     |  Unknown                  |
        
|  Leon       |  Unknown                  |
        
+-----------+----------------------+
        
8 rows in set (0.00 sec)
       
       
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attach 、 A picture with a story （ Nine ）

MySQL Summary of common judgment functions ！！ See if you've used it _ data _02

This picture is from 08 In Wenchuan ;
The man who lost his wife in the earthquake is full of deep feelings ,
I can't bear to leave my wife in the wild ,
Bind his body to himself ,
Take her on a motorcycle to the local morgue .
In the agony of great grief ,
He's still trying to give his wife some dignity after death .

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