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Summary of MySQL common judgment functions!! Have you used it
2022-07-04 07:01:00 【Chen haha Ben ha】
Partners want to find out exactly what they want to see MySQL article ？ here → MySQL Jianghu Road | Column catalog
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 ？
Catalog
- 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 ）
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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