Question

我有一个SELECT语句，它使用LEFT JOIN从我的数据库中的不同表中获取各种字段。我只想在满足某个条件时选择其他字段。这是我尝试实现的一个例子。

boolean return_flag = list_name.contains(var_name)
if return_flag{
    //do stuff
}

为了澄清，上面的例子已经完全弥补了，但它展示了我尝试做的事情。我知道＆＃39; IF＆＃39;中间的陈述是完全错误的 - 这是我需要帮助的一点。我查看了MySQL IF语句和MySQL IF函数，但无法弄清楚它应该如何工作。

任何帮助都非常感激。

谢谢！

Answer 1

IF子句喜欢这个;）

SELECT table1.useriD, 
       table1.name, 
       table2.address, 
       table2.employed, 
       IF(table2.employed = 1, table3.jobTitle, null) AS additionalCol
FROM table1 
LEFT JOIN table2 ON table1.userID = table2.userID
LEFT JOIN table3 ON table2.userID = table3.userID

您也可以使用CASE WHEN：

SELECT table1.useriD, 
       table1.name, 
       table2.address, 
       table2.employed, 
       CASE WHEN table2.employed = 1 THEN table3.jobTitle ELSE null END AS additionalCol
FROM table1 
LEFT JOIN table2 ON table1.userID = table2.userID
LEFT JOIN table3 ON table2.userID = table3.userID

Answer 2

SELECT table1.useriD, 
       table1.name, 
       table2.address, 
       table2.employed, 
       (IF table2.employed = 1, table3.jobTitle, null) as JobTitle
FROM table1 
LEFT JOIN table2 ON table1.userID = table2.userID
LEFT JOIN table3 ON table2.userID = table3.userID

参考：http://dev.mysql.com/doc/refman/5.7/en/control-flow-functions.html

Answer 3

你的if条件应该如下

SELECT table1.useriD, 
   table1.name, 
   table2.address, 
   table2.employed, 
   (IF table2.employed = 1, table3.jobTitle, null) as jobTitle
FROM table1 
LEFT JOIN table2 ON table1.userID = table2.userID
LEFT JOIN table3 ON table2.userID = table3.userID

如何在MySQL语句中有条件地选择某些字段

3 个答案: