oracle多表查询优化

用select 时，少用* 多用字段

select name,code from table, ;-------------------select * from table

select  count(*) from table1 ,tabale2--------------table2的记录数比table1的记录少，查询速度快

如果要是三张表，其中一个是中间表的话，中间表放在最右边，

少用in，多用exsits

、少用in，多用exists来代替

--NO.1 IN的写法
SELECT * FROM TABLEA A WHERE
A.ID IN (SELECT ID FORM TABLEB B WHERE B.ID>1)

--NO.2 exists 写法
SELECT * FROM TABLEA A WHERE
EXISTS (SELECT 1 FROM TABLEB B WHERE A.ID=B.ID AND B.ID>1)

select * from t_c04_county  a where exists (select 1 from t_c03_city b where a.ys_city_code=b.ys_city_code )