mysql - 使用 Exists 时，我应该对多个子查询使用 OR 还是对所有子查询使用 UNION？

Question

我正在编写一个查询，根据其他表中满足的某些条件来确定数据库中是否存在记录。

现在我想出了一种情况，我需要知道什么是最好的方法:

或者子查询:

SELECT * from table as tbl WHERE
Exists
(SELECT 1 FROM t1 WHERE t1.id = tbl.id)
OR Exists
(SELECT 1 FROM t2 WHERE t2.col2 = tbl.col2)
OR Exists
(SELECT 1 FROM t3 WHERE t3.othercol = tbl.somecol)

或使用不带 OR 的 UNION:

SELECT * from tbl WHERE
Exists
(SELECT 1 FROM t1 WHERE t1.id = tbl.id
 UNION
 SELECT 1 FROM t2 WHERE t2.col2 = tbl.col2)
 UNION
 SELECT 1 FROM t3 WHERE t3.othercol = tbl.somecol)

我需要它是最好的性能，因此问题。 一些字段/列可能没有被索引，这可能发生在一组不同的列中，不仅仅是 3 个，甚至每个子查询/表可能超过 1 个。

为了获得最佳解决方案，我将在此处发布一些复杂的示例: 如果加入连接会怎样？

SELECT * from table as cli WHERE
Exists 
        (

        SELECT 
            1
        from 
            tbl_import_line_reference as l,
            tbl_import_doc as d
        WHERE
            d.import_key = l.import_key AND
            CAST(Left(d.doc_date,8) as  DATE) BETWEEN  LAST_DAY(NOW() - INTERVAL 6 MONTH) + INTERVAL 1 day AND  NOW()
        AND
            l.prod_ref like '---fmidref%'
        AND
            d.doc_type ='F'
        AND 
            d.car_miposreg_ext_id = cli.car_miposreg_ext_id
        )

    OR
    Exists
        (

        select 
            1
        from 
            tbl_import_line_reference as l,
            tbl_import_doc as d
        WHERE
            d.import_key = l.import_key AND
            CAST(Left(d.doc_date,8) as  DATE) BETWEEN  @data_final + INTERVAL 1 day AND  NOW()
        AND
            l.prod_ref not RLIKE '---fmidneo|---fmidevo'
        AND
            l.act_code = 5
        AND
            l.act_subcode = "7"
        AND
            d.doc_type ='F'
        AND 
            d.car_miposreg_ext_id = cli.car_miposreg_ext_id
        )

之后是:http://pastebin.com/gTFBFurV变成了这个:http://pastebin.com/y13xKcMg

Answer 1

通过使用左连接，它仅使用索引(前提是您的表在每个相应的“ID”、“col2”和“othercol”基础上都有索引。索引非常快，不需要去原始确认它“存在”或不存在的页面。where 子句只关心任何元素 NOT NULL(表明它确实存在于基础表中)

SELECT 
      tbl.* 
   from 
      table as tbl
         LEFT JOIN t1
            ON tbl.id = t1.id
         LEFT JOIN t2
            ON tbl.col2 = t2.col2
         LEFT JOIN t3
            ON tbl.somecol = t3.othercol
   WHERE
         t1.ID is not null
      OR t2.col2 is not null
      OR t3.othercol is not null