ENexists用法 exists: 如果括号内子查询语句返回结果不为空,说明where条件成立,就会执行主SQL语句 如果括...
select count(*) from db_test.t_zh_axx aj where exists (select 1 from db_test.t_zh_zjxx zbajxx where zbajxx.c_ajbh=aj.c_ajbh and c_zblx = '0020002') and aj.c_dbbh = '8B7D8C93864E0D0C3E3259C49ED65471' AND aj.c_zy = '1801' --执行计划都走索引 --值得关注的是join...
postgresql exists用法 PostgreSQL的EXISTS用法可以用于检查子查询中是否存在结果。它返回一个布尔值,如果子查询返回至少一个行,则为true;否则为false。 以下是EXISTS用法的示例: 1.检查子查询中是否存在结果: SELECT column_name FROM table_name WHERE EXISTS (SELECT column_name FROM other_table WHERE condition);...
select d.department_id, d.department_name from departments d where not exists (select 1 from employees where department_id = d.department_id); 以上语句查找没有任何员工的部门,结果返回了 16 条记录。如果使用NOT IN操作符: select d.department_id, d.department_name from departments d where d.dep...
EXISTS 本身是循环外表,简则内表的行是否在外表中存在 我们下面先入为主的用三查询来说明 select sum(pay.amount),sta.staff_id from payment as pay left join staff as sta on pay.staff_id = sta.staff_id left join (select rental_date,rental_id from rental where rental_date > '2000-09-08')...
select * from A where id in (select id from B); select * from A where exists (select 1 from B where =); 对于以上两种情况,in是在内存里遍历比较,而exists需要查询数据库,所以当B表数据量较大时,exists效率优于in。 1、select * from A where id in (select id from B); ...
create table exists_01 (id int,name varchar(12),des text); insert into exists_01 values (1,'Tom','ms1'),(2,'Jer','ms2'),(3,'Jim','es1'); ``` 如何筛选出des是ms系列的角色? ① ``` pgadv=# select name from exists_01 where des like 'ms%'; ...
postgres=# explain (analyze,verbose,timing,costs,buffers) select *froma where exists (select 1fromb whereinfo~~'%abc%'anda.id=b.aid); QUERY PLAN --- Merge Join (cost=23102.75..23103.04rows=10width=13) (actualtime=74.316..74.325rows=9loops=1) Output: a.id, a.xx Inner Unique:trueMer...
1 2 3 4 5 6 7 8 9 SELECT count( aid ),a.bid FROM pgbench_accounts a JOIN pgbench_branches b ON a.bid = b.bid WHERE b.bbalance > 0 GROUP BY a.bid; 在完成这个查询要求的时候,有人可能会假设exists和inner join性能可能会更好,因为他们可以使用两表连接的逻辑和优化。而IN和ANY子句...
postgresql中,许多ddl语句支持if exists、if not exists。例如: 1 2 3 4 5 postgres=#createtableifnotexists abce(); CREATETABLE postgres=#droptableif exists abce; DROPTABLE postgres=# 建议只是在必须的时候在ddl中使用if exists、if not exists。以下是三个示例,展示了过度使用他们而产生的负面效应。