Question

我想在mysql select中使用IF THEN语句，但无法弄明白。当还没有评论时，评论创建的值应该是项目本身的创建值。这是查询：

SELECT item.*, 
  count(comments.itemid) AS commentcount, 
  max(comments.created) AS commentcreated 
FROM table_items AS item 
LEFT JOIN table_comments AS comments ON (comments.itemid = item.id) 
WHERE item.published=1 
GROUP BY item.id 
ORDER BY item.created DESC

我想到了这一点，但它不起作用：

...
, IF max(comments.created)!='NULL'
THEN max(comments.created) AS commentcreated
ELSE item.created AS commentcreated
END IF;
FROM .......

最好的方法是什么？

Answer 1

可以使用IFNULL

IFNULL(max(comments.created), item.created) AS commentcreated

或IF

IF(max(comments.created)='NULL', max(comments.created), item.created) 
AS commentcreated

Answer 2

coalesce(max(comments.created), item.created) as commentcreated

Answer 3

你不能在单个语句中使用IF ... THEN ... ELSE构造，作为表达式的一部分来产生单个值 - 这就是IFNULL和COALESCE函数的用途，如ajreal和Redfilter建议。

IF ...那么...... ELSE仅适用于包含许多不同语句的SQL代码块，例如：在存储过程中。

MySql如果然后在Select语句中

3 个答案: