如何在MySQL查询中编写IF ELSE语句

Question

如何在MySQL查询中编写IF ELSE语句？

像这样的东西:

mysql_query("...(irrelevant code).. IF(action==2&&state==0){state=1}");

然后在我的数组中我应该能够这样做:

 $row['state'] 
//this should equal 1, the query should not change anything in the database, 
//just the variable for returning the information

Answer 1

您可能想要使用CASE表达式.

它们看起来像这样:

SELECT col1, col2, (case when (action = 2 and state = 0) 
 THEN
      1 
 ELSE
      0 
 END)
 as state from tbl1;

Answer 2

你必须用SQL写它而不是C/PHP风格

IF( action = 2 AND state = 0, 1, 0 ) AS state

用于查询

IF ( action = 2 AND state = 0 ) THEN SET state = 1

用于存储过程或函数

Answer 3

您正在寻找case:

case when action = 2 and state = 0 then 1 else 0 end as state

MySQL有一个if语法(if(action=2 and state=0, 1, 0)),但case更通用.

请注意,as state只有列的别名.我假设这是在您的SQL查询的列列表中.

Answer 4

SELECT col1, col2, IF( action = 2 AND state = 0, 1, 0 ) AS state from tbl1;

要么

SELECT col1, col2, (case when (action = 2 and state = 0) then 1 else 0 end) as state from tbl1;

两个结果都一样....

Answer 5

根据mySQL参考手册,这个使用if和else语句的语法:

IF search_condition THEN statement_list
[ELSEIF search_condition THEN statement_list] ...
[ELSE statement_list]
END IF

所以关于你的查询:

x = IF((action=2)&&(state=0),1,2);

或者你可以使用

IF ((action=2)&&(state=0)) then 
state = 1;
ELSE 
state = 2;
END IF;

这个链接有很好的例子:http: //easysolutionweb.com/sql-pl-sql/how-to-use-if-and-else-in-mysql/