在别名HAVING和不使用HAVING之间有性能差异 [英] Is there a performance difference between HAVING on alias, vs not using HAVING

问题描述

好，我在学习，位 位，关于<$现在，我的问题是，如果这两个查询有不同的性能特点：

b

没有HAVING

  SELECT x + y AS z，t。 * FROM t 
 WHERE 
x = 1 and 
 x + y = 2 
  

包含HAVING

  SELECT x + y AS z，t。* FROM t 
 WHERE 
x = 1 
 HAVING 
z = 2 
  

解决方案

HAVING用于包含GROUP BY的查询，或返回包含聚合函数结果的单个行。例如 SELECT SUM（score）FROM t HAVING SUM（scores）> 100

$

第二个查询被SQL标准视为无效，并且不被某些数据库系统。

Ok, I'm learning, bit by bit, about what HAVING means.

Now, my question is if these two queries have difference performance characteristics:

Without HAVING

SELECT x + y AS z, t.*  FROM t
WHERE 
x = 1 and
x+y = 2

With HAVING

SELECT x + y AS z, t.*  FROM t
WHERE 
x = 1
HAVING
z = 2

解决方案

HAVING is used for queries that contain GROUP BY or return a single row containg the result of aggregate functions. For example SELECT SUM(scores) FROM t HAVING SUM(scores) > 100 returns either one row, or no row at all.

The second query is considered invalid by the SQL Standard and is not accepted by some database systems.

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