Question

给定两个子查询表t1和t2，当且仅当t1返回空行时，如何返回t2？

修改：添加了示例

T1

SELECT * 
  FROM common_table 
 WHERE language_id = 1

T2

SELECT * 
  FROM common_table 
 WHERE language_id = 2

基本上我正在做的是，如果T1返回空行，我希望它执行T2并返回这些行。现在，我完全清楚我可以在PHP中执行此操作，但查询是一个子查询，我宁愿让SQL（ 不是PHP ）代码处理它。

Answer 1

Select ...
From common_table
Where language_id = 1
    Or  (
        language_id = 2
        And Not Exists  (
                        Select 1
                        From common_table
                        Where language_id = 1
                        )
        )

Answer 2

在MSSQL中我可以做类似

的事情

If EXISTS (SELECT * FROM common_table WHERE language_id = 1)
BEGIN
    SELECT * FROM common_table WHERE language_id = 1
END ELSE BEGIN
    SELECT * FROM common_table WHERE language_id = 2
END

在mySQL或类似的

中应该是相同的

Answer 3

我的MySql知识有点生疏，我认为这应该有用。

SELECT * FROM common_table 
         WHERE language_id = 2 and 
               0 = ( SELECT count(*) FROM common_table 
                                     WHERE language_id = 1
                   ) ;

Answer 4

SELECT * 
  FROM common_table 
 WHERE language_id = 1
UNION
SELECT * 
  FROM common_table 
 WHERE language_id = 2
       AND NOT EXISTS (
                       SELECT * 
                         FROM common_table AS T2 
                        WHERE T2.language_id = 1
                      );

Mysql - 当且仅当第一个子查询返回空时，才使用辅助子查询

4 个答案: