mysql - 使用合并或 if 导致 where 子句 mysql

Question

我使用一些这样的sql：

SELECT COALESCE(group.display,item.display) as display....

我想在 WHERE 子句中添加：

WHERE display='1'

WHERE 显示是合并的结果。

同样，我希望能够对这样的事情做同样的事情：

IF(ISNULL(gd.group_main_image),p.main_image,gd.group_main_image) AS image
... WHERE image IS NOT NULL

请问我该怎么做？

Answer 1

您不能在同一级别的查询中使用别名。

你必须重复自己。

WHERE COALESCE(group.display,item.display) = '1'

编辑

好吧，我限制得太多了。您可以在 MySql 的 having 子句中使用别名。您不能在其他 DBMS（Oracle、SQl Server）中做到这一点。通常它在 ANSI SQL 中也是不允许的。

Answer 2

As described in Problems with Column Aliases:

An alias can be used in a query select list to give a column a different name. You can use the alias in GROUP BY, ORDER BY, or HAVING clauses to refer to the column:

SELECT SQRT(a*b) AS root FROM tbl_name
  GROUP BY root HAVING root > 0;
SELECT id, COUNT(*) AS cnt FROM tbl_name
  GROUP BY id HAVING cnt > 0;
SELECT id AS 'Customer identity' FROM tbl_name;

Standard SQL disallows references to column aliases in a WHERE clause. This restriction is imposed because when the WHERE clause is evaluated, the column value may not yet have been determined. For example, the following query is illegal:

SELECT id, COUNT(*) AS cnt FROM tbl_name
  WHERE cnt > 0 GROUP BY id;

The WHERE clause determines which rows should be included in the GROUP BY clause, but it refers to the alias of a column value that is not known until after the rows have been selected, and grouped by the GROUP BY.

Answer 3

您不能在子句中使用列别名WHERE

所以：

WHERE COALESCE(group.display,item.display)='1'

或者：

HAVING display='1'

但是，HAVING在发现所有结果集之后执行，所以基本上这会更消耗内存