SQL字段拆分优化

今天看到一条用函数处理连接的SQL，是群里某位网友的，SQL语句如下：

SELECT SO_Order.fdate ,  SO_Order.fsnFROM so_order  INNER JOIN so_orderitem ON CHARINDEX(so_Orderitem.fid, SO_Order.fid) >= 1WHERE so_order.FOrderDate = '2015-09-06'

--1.SQL执行203条数据

--2.耗时12秒

--3.so_order表的fid字段是字符串集合，

--由1-2个字符串组成,用','分隔

SELECT COUNT(*)FROM SO_OrderWHERE so_order.FOrderDate = '2015-09-06'--24SELECT COUNT(*)FROM so_Orderitem--414154

扫描上。因此，我们要通过改写SQL，达到so_orderitem走索引查找的目的。

　　将so_order表的fid字段拆分成2个字段，然后union all成一个结果集，再和so_orderitem关联，即可让so_orderitem表走索引查找。

改写的SQL如下：

;WITH x0   AS ( SELECT fdate ,      fsn ,      LEFT(fid, CASE WHEN CHARINDEX(',', fid, 1) = 0 THEN 0          ELSE CHARINDEX(',', fid, 1) - 1         END) AS fid    FROM  so_order    WHERE FOrderDate = '2015-09-06'    UNION ALL    SELECT fdate ,      fsn ,      RIGHT(fid, LEN(fid) - CHARINDEX(',', fid, 1)) AS fid    FROM  so_order    WHERE FOrderDate = '2015-09-06'    ) SELECT SO_Order.fdate ,   SO_Order.fsn FROM x0 so_order   INNER JOIN so_orderitem ON so_Orderitem.fid = SO_Order.fid

　　改写后SQL的执行计划如下：

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