在sql语句中经常使用到多表组合查询
在多表组合查询中有时会用将一个查询结果集用括号括起来放在from语句后面当临时表用，
如果一个组合查询用有很多将一个查询结果集用括号括起来放在from语句后面当临时表用，或者from语句多成嵌套，或者同一结果集多次出现的情况，
建议使用Oracle with语句创建一个公共临时表。因为子查询在内存临时表中，避免了重复解析，所以执行效率会提高不少。临时表在一次查询结束自动清除。
WITH语句的优点:
SQL可读性增强。
with子查询只执行一次，将结果存储在用户临时表空间中，可以引用多次，增强性能。
分层次查询，一步步筛选出结果集。
一般语法格式:
with
alias_name1 as (subquery1),
alias_name2 as (subQuery2),
……
alias_nameN as (subQueryN)
select col1,col2…… col3
from alias_name1,alias_name2……,alias_nameN
在一个表的基础上利用WITH语句创建多个临时表实现复杂的逻辑查询
WITH AMCH1 AS ( SELECT ACC_NUM, MAX (BUSINESS_DATE) AS MAX_BUSINESS_DATE
FROM ACC_MARGIN_CALL_HIST_TBL
GROUP BY ACC_NUM),
AMCH2
AS (SELECT T.ACC_NUM AS ACC_NUM,
T.BUSINESS_DATE AS MARGIN_CALL_SETTLE_DATE,
T.CALL_AMT_START_DATE AS MARGIN_CALL_START_DATE,
T.NUM_OF_DAYS_IN_PREC_DAYS AS NUM_OF_DAYS_IN_PREC_DAYS,
T.NUM_OF_LAST_LONG_CALL_DAYS
AS NUM_OF_LAST_LONG_CALL_DAYS,
NVL (
( SELECT COUNT (ACC_NUM)
FROM ACC_MARGIN_CALL_HIST_TBL A
WHERE A.ACC_NUM = T.ACC_NUM
AND A.BUSINESS_DATE BETWEEN (SELECT TO_CHAR (
ADD_MONTHS (
TO_DATE (
(SELECT bo_system_pkg.
getbusinessdate
FROM DUAL),
'yyyymmdd'),
-1),
'yyyymmdd')
FROM DUAL)
AND (SELECT bo_system_pkg.
getbusinessdate
FROM DUAL)
GROUP BY A.ACC_NUM),
0)
AS POOR_CREDIT_FLAG_COUNT
FROM ACC_MARGIN_CALL_HIST_TBL T, AMCH1
WHERE T.ACC_NUM = AMCH1.ACC_NUM
AND T.BUSINESS_DATE = AMCH1.MAX_BUSINESS_DATE),
AMCH3
AS (SELECT T.ACC_NUM AS ACC_NUM,
CASE
WHEN (AMCH2.POOR_CREDIT_FLAG_COUNT < 15
AND T.POOR_HISTORY_FLAG = 'N')
OR (AMCH2.POOR_CREDIT_FLAG_COUNT >= 15
AND T.POOR_HISTORY_FLAG = 'N')
THEN
NULL
ELSE --T.BUSINESS_DATE
( SELECT CASE
WHEN (MAX (BUSINESS_DATE) =
(SELECT bo_system_pkg.
get_pre_business_date (
bo_system_pkg.
getbusinessdate,
1,
'HK',
'H')
AS prev_dt
FROM DUAL))
THEN
(SELECT bo_system_pkg.
getbusinessdate
FROM DUAL)
ELSE
MAX (BUSINESS_DATE)
END
FROM ACC_MARGIN_CALL_HIST_TBL
WHERE POOR_HISTORY_FLAG = 'Y'
AND NUM_OF_DAYS_IN_PREC_DAYS = 29
AND NUM_OF_CALL_DAYS >= 91
AND ACC_NUM = AMCH2.ACC_NUM
GROUP BY ACC_NUM)
END
AS MARGIN_CALL_SETTLE_DATE,
CASE
WHEN (AMCH2.POOR_CREDIT_FLAG_COUNT < 15
AND T.POOR_HISTORY_FLAG = 'N')
OR (AMCH2.POOR_CREDIT_FLAG_COUNT >= 15
AND T.POOR_HISTORY_FLAG = 'N')
THEN
NULL
ELSE --T.CALL_AMT_START_DATE
( SELECT MAX (CALL_AMT_START_DATE)
FROM ACC_MARGIN_CALL_HIST_TBL
WHERE POOR_HISTORY_FLAG = 'Y'
AND NUM_OF_DAYS_IN_PREC_DAYS = 29
AND NUM_OF_CALL_DAYS >= 91
AND ACC_NUM = AMCH2.ACC_NUM
GROUP BY ACC_NUM)
END
AS MARGIN_CALL_START_DATE,
CASE
WHEN AMCH2.POOR_CREDIT_FLAG_COUNT >= 15
OR T.POOR_HISTORY_FLAG = 'Y'
THEN
'Y'
ELSE
'N'
END
AS POOR_CREDI,
AMCH2.POOR_CREDIT_FLAG_COUNT AS POOR_CREDIT_FLAG_COUNT
FROM ACC_MARGIN_CALL_HIST_TBL T, AMCH2
WHERE T.ACC_NUM = AMCH2.ACC_NUM
AND T.BUSINESS_DATE = AMCH2.MARGIN_CALL_SETTLE_DATE),
AMCH4
AS (SELECT G.NICK_NAME AS CLIENT_ACC_ID,
DECODE (A.ACC_TYPE, 'C', 'N', 'M', AMCH3.POOR_CREDI)
AS POOR_CREDIT,
0 AS TRANSFER_AMOUNT,
0 AS TRANSFER_RATIO,
DECODE (A.ACC_TYPE,
'C', 0,
'M', AMCH3.POOR_CREDIT_FLAG_COUNT)
AS POOR_CREDIT_FLAG_COUNT,
DECODE (A.ACC_TYPE,
'C', NULL,
'M', AMCH3.MARGIN_CALL_START_DATE)
AS MARGIN_CALL_START_DATE,
DECODE (A.ACC_TYPE,
'C', NULL,
'M', AMCH3.MARGIN_CALL_SETTLE_DATE)
AS MARGIN_CALL_SETTLE_DATE,
NULL AS EXEMPTION_END_DATE,
NULL AS EXEMPTION_REASON
FROM AMCH3,
( SELECT SUBSTR (ACC_NUM, 1, 10) AS ACC_NUM,
MAX (ACC_TYPE) AS ACC_TYPE,
MAX (ACC_GROUP) AS ACC_GROUP
FROM ACC_TBL
GROUP BY SUBSTR (ACC_NUM, 1, 10)
HAVING MAX (ACC_TYPE) IN ('C', 'M')) A,
(SELECT ACC_GROUP, NICK_NAME FROM ACC_GRP_TBL) G
WHERE AMCH3.ACC_NUM = A.ACC_NUM AND A.ACC_GROUP = G.ACC_GROUP)
SELECT V.CLIENT_ACC_CODE AS CLIENT_ACC_ID,
NVL (A.POOR_CREDIT, 'N') AS POOR_CREDIT,
NVL (A.TRANSFER_AMOUNT, 0) AS TRANSFER_AMOUNT,
NVL (A.TRANSFER_RATIO, 0) AS TRANSFER_RATIO,
NVL (A.POOR_CREDIT_FLAG_COUNT, 0) AS POOR_CREDIT_FLAG_COUNT,
A.MARGIN_CALL_START_DATE AS MARGIN_CALL_START_DATE,
A.MARGIN_CALL_SETTLE_DATE AS MARGIN_CALL_SETTLE_DATE,
NVL (A.EXEMPTION_END_DATE, 0) AS EXEMPTION_END_DATE,
NVL (A.EXEMPTION_REASON, 0) AS EXEMPTION_REASON
FROM EXP_CLIENT_ACC_CODE_V V,
( SELECT CLIENT_ACC_ID,
MAX (POOR_CREDIT) AS POOR_CREDIT,
MAX (TRANSFER_AMOUNT) AS TRANSFER_AMOUNT,
MAX (TRANSFER_RATIO) AS TRANSFER_RATIO,
MAX (POOR_CREDIT_FLAG_COUNT) AS POOR_CREDIT_FLAG_COUNT,
MAX (MARGIN_CALL_START_DATE) AS MARGIN_CALL_START_DATE,
MAX (MARGIN_CALL_SETTLE_DATE) AS MARGIN_CALL_SETTLE_DATE,
MAX (EXEMPTION_END_DATE) AS EXEMPTION_END_DATE,
MAX (EXEMPTION_REASON) AS EXEMPTION_REASON
FROM AMCH4
GROUP BY CLIENT_ACC_ID) A
WHERE V.CLIENT_ACC_CODE = A.CLIENT_ACC_ID(+);