sql優化日記2（join代替in）

EXPLAIN
SELECT count(*)
FROM emp
WHERE EXISTS (SELECT *
FROM dep

WHERE dep.depid=emp.depid)

EXPLAIN
SELECT count(*)
FROM emp
WHERE depid in (SELECT depid
FROM dep
)

EXPLAIN 
SELECT count(*)
FROM emp join dep on emp.depid=dep.depid

EXPLAIN 
SELECT count(*)
FROM emp , dep 
WHERE  emp.depid=dep.depid

https://www.cnblogs.com/rising-fay/archive/2012/11/13/2767538.html