1) with as
a) WITH AS短语,也叫做子查询部分(subquery factoring),可以定义一个SQL片断,该SQL片断会被整个SQL语句所用到。
b) 当查询中多次用到某一部分时,可以用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。
c) 应用:需多次调用;union all中;
d) 示例
with
sql1 as (select s_name from test_tempa),
sql2 as (select s_name from test_tempb where not exists (select s_name from sql1))
select * from sql1
union all
select * from sql2
union all
select 'no records' from dual
where not exists (select s_name from sql1 where)
and not exists (select s_name from sql2);
2) case when
a) 简单case语句
select case to_char(sysdate,’d’)
when ‘1’then ‘周日’
when ‘2’ then ‘周一’
else ‘不知道’
end
from dual;
b) 搜索case语句
SELECT count(col3), sum(CASE WHEN col3 = 1 THEN 1
ELSE 0
END) ,
sum(CASE WHEN col3 = 2 THEN 1
ELSE 0
END)
FROM table_name;
c) where case when 用法
SELECT T2.*, T1.*
FROM T1, T2
WHERE (CASE WHEN T2.col2 = 'A' AND
T1.col3 = 'B'
THEN 1
WHEN T2.col2 != 'A' AND
T1.col3 != 'B'
THEN 1
ELSE 0
END) = 1
d) group by case when 用法
SELECT CASE
WHEN col2 = '05' THEN '1'
WHEN col2 = '23' THEN '2'
ELSE NULL END "类别",
COUNT(*)
FROM table_name
GROUP BY CASE
WHEN col2 = '05' THEN '1'
WHEN col2 = '23' THEN '2'
ELSE NULL END;
3) merge into
在oracle 11g中有如下特点
a) update和insert子句时可选的。
b) update和insert子句可以加where子句
merge into table_a a
using table_b b
on (a.id = b.id)
when matched then
update set a.name = b.name where ……
when not matched then
insert values (a.id,a.name) where ……
4) 分析函数over()
a) ROW_NUMBER(),不允许并列名次、相同值名次不重复,结果如123456……
b) RANK(),跳跃排序,允许并列名次、复制名次自动空缺,结果如12245558……
c) DENSE_RANK(),允许并列名次、名次不间断,结果如122344456……
d) 例子:
select * from (
selelct row_number() over(partition by xxx2 order by xxx3 ) rw from dual)tt where tt.rw = ?;