尝试运行IF EXISTS查询时出现语法错误

Question

我正在尝试运行以下查询（在Sequel Pro中），但我收到以下语法错误：

 '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

您不能在函数之外使用if块。而是只运行后续的SQL语句：

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

如果没有testobserver值的用户名，则它本身就不会返回任何内容。