使用`from A内部联接B ... LIMIT x，y`进行MySQL查询是否在将y行从表A中取出之前或之后进行联接? [英] Does a MySQL query with `from A inner join B ...LIMIT x,y` do the join before or after getting the y rows out of table A?

问题描述

对于下面的以下命令

1. MySQL在从中获取100行之前或之后执行联接吗 表A?

2. 是否可以重写查询以获取行 在进行联接之前从表A中获取数据?

1. Does MySQL do the joins before or after getting the 100 rows from the table A?

2. Is there a way to rewrite the query to get the rows from table A before the joins are made?

SELECT
   A.START_TIME, A.F1, A.F2, B.STRING
FROM    
   A INNER JOIN B ON A.B_ID=B.ID
WHERE
   A.START_TIME>= '2015-03-22 05:23:44'
LIMIT 0, 100;

推荐答案

1. 否，在合并的数据集上的连接后后应用限制.
2. 如果只想在联接之前对表A施加限制，则需要使用子查询:

1. No, the limit is applied after the join on the combined dataset.
2. If you want to apply the limit on table A only before the join, then you need to use a subquery:

SELECT T.START_TIME, T.F1, T.F2, B.STRING FROM
(select A.START_TIME, A.F1, A.F2 from A WHERE A.START_TIME>= '2015-03-22 05:23:44' limit 0, 100) T INNER JOIN B ON T.B_ID=B.ID;

SELECT T.START_TIME, T.F1, T.F2, B.STRING FROM
(select A.START_TIME, A.F1, A.F2 from A WHERE A.START_TIME>= '2015-03-22 05:23:44' limit 0, 100) T INNER JOIN B ON T.B_ID=B.ID;

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