语法错误EXISTS查询

Question

我试图运行下面的查询（在续集专业），但我得到了以下语法错误：

'You have an error in your SQL syntax; check the manual that corresponds to 
    your MySQL server version for the right syntax to use near 'IF 
    (EXISTS(SELECT username FROM USER WHERE username = "TestObserver")) 
    {SELECT *' at line 1' 

代码：

IF EXISTS (SELECT username FROM USER WHERE username = "TestObserver") 
SELECT * FROM USER WHERE username = "TestObserver" 

Answer 1

您不能在功能以外使用if块。而不是仅仅运行后续的SQL语句：

SELECT * FROM `USER` WHERE username = "TestObserver" 

如果没有与testobserver值会已经全部由自己返回任何没有用户名。