同事写了一个逻辑稍复杂的Update 语句，觉得在代码可读性上有些转圈，交给我帮忙改下。
以下根据原SQL，使用两种方法进行改写，个人推荐方法1的改写。方法2拆分两个SQL来写，代码可读性最强，但是SQL扩展上不利，推荐方法1的改写.
原SQL：
UPDATE z_delivery_orders a
SET dtheday2 = a.dtheday2 +
(SELECT c.add_dt
FROM ud_cfg_firmdt c
WHERE c.add_inv_type = (CASE WHEN
a.VBELN IN (SELECT VBELN FROM Z_MID_PCDW_SO b WHERE b.zcustseg = 'WEB')
THEN 'A'
ELSE
'B'
END)) )
WHERE a.dtheday2 IS NOT NULL;
update SQl 本身是没有问题的，逻辑也是符合要求，且不说表的设计上是否合理，单从SQL的改写上着手做增强。
改写1：
MERGE INTO z_delivery_orders a
USING (select rowid as row_id,
(CASE
WHEN VBELN IN (SELECT VBELN
FROM Z_MID_PCDW_SO b
WHERE b.zcustseg = 'WEB') THEN
'A'
ELSE
'B'
END) as type_1
from z_delivery_orders
WHERE dtheday2 IS NOT NULL ) b
ON (a.rowid = b.row_id)
WHEN MATCHED THEN
UPDATE
SET a.dtheday2 =
(SELECT a.dtheday2 + nvl(max(add_dt), 0) as dtheday2
FROM ud_cfg_firmdt
WHERE add_inv_type = b.type_1);
改写2：拆分两个SQL来写，代码可读性最强，但是SQL扩展上不利，推荐方法1的改写
UPDATE z_delivery_orders a
SET dtheday2 = dtheday2 +
(SELECT add_dt
FROM ud_cfg_firmdt
WHERE add_inv_type = ’A'
and rownum = 1)
WHERE dtheday2 IS NOT NULL
AND exists (select 1
from Z_MID_PCDW_SO b
WHERE b.zcustseg = 'WEB'
and a.vbeln = b.vbeln);
UPDATE z_delivery_orders a
SET dtheday2 = dtheday2, +
(SELECT add_dt
FROM ud_cfg_firmdt
WHERE add_inv_type = 'B'
and rownum = 1)
WHERE dtheday2 IS NOT NULL
AND not exists (select 1
from Z_MID_PCDW_SO b
WHERE b.zcustseg = 'WEB'
and a.vbeln = b.vbeln)