標量子查詢與改寫邏輯的一致性
當標量子查詢中有聚合函式時，很多人在改寫時，很喜歡把聚合函式寫在主查詢裡，大概是認為這樣簡單。
select *
from (select * form t1 where col4_1 = '0') a
left
join (select col3_1,
col3_2 coll,
col2,
select
distinct col_1
from t2
where col2_2 = a.col2
and col2_3 <= to_date('2012-09-03','yyyy-mm-dd')
and col2_4 >= to_date('2012-09-03','yyyy-mm-dd'))
from t3 a
where col2_3 <= to_date('2012-09-03','yyyy-mm-dd')
and col2_4 >= to_date('2012-09-03','yyyy-mm-dd'))b
on a.col4 = b.col3_1;
網友在改寫這個查詢時，直接把distinct 放在了主查詢select關鍵字後面
select *
from (select * form t1 where col4_1 = '0') a
left
join (select
distinct a.col3_1,
a.col3_2 coll,
a.col2,
b.col2_1 as col3
from t3 a
join (select *
from t2 b
where b.col2_3 <= to_date('2012-09-03','yyyy-mm-dd')
and b.col2_4 >= to_date('2012-09-03','yyyy-mm-dd')) b
on a.col2 =b.col2_2
and a.col2_3 <= to_date('2012-09-03','yyyy-mm-dd')
and a.col2_4 >= to_date('2012-09-03','yyyy-mm-dd')) b
on a.col4 = b.col3_1;
原查詢可以表示為：
select a.job,
a.dept,
(select
distinct dname from dept2 b where b.deptno = a.deptno) as dename,
from emp a
order
by1,2,3;
按上面的方法更改：
select
distinct a.job,
a.deptno,
b.dname
from emp a
left
join dept2 b on b.deptno = a.deptno;
除非確認主查詢沒有重複資料，否則不要這樣改寫，正確的改寫如下：
select a.job,
a.deptno,
b.dname
from emp a
left
join (select dname,deptno from dept2 group
by dname,deptno) b
on b.deptno = a.deptno;
SQL查詢和優化（三）
給查詢結果排序 select empno,ename,hredate from emp where deptno 10 order by hiredate asc 也可以這樣寫 select empno,ename,hredate from emp where deptno 10 orderby3 ...
SQL查詢和優化（五）
一 插入新記錄 建立測試表，各列都有預設值。create table test c1 varchar2 10 defaut 預設1 c2 varchar2 10 defaut 預設2 c3 varchar2 10 defaut 預設3 c4 date default sysdate 新增資料如下 i...
SQL查詢和優化（九）
用left join 優化標量子查詢 多次訪問同乙個表時，盡量不用標量子查詢 select s.sid,s.sname,s.shot,s.stype,select a.aid from a where a.aid s.aids aid,select a.aname from a where a.ai...