MySQL中in与exists的使用及区别介绍

for(int i=0;i<1000;i++){
 for(int j=0;j<5;j++){
 System.out.println("hello");
 }
}

 for(int i=0;i<5;i++){
 for(int j=0;j<1000;j++){
 System.out.println("hello");
 }
}

mysql> desc tbl_emp;
+--------+-------------+------+-----+---------+----------------+
| Field | Type  | Null | Key | Default | Extra   |
+--------+-------------+------+-----+---------+----------------+
| id  | int(11)  | NO | PRI | NULL | auto_increment |
| name | varchar(20) | YES |  | NULL |    |
| deptld | int(11)  | YES | MUL | NULL |    |
+--------+-------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)

mysql> desc tbl_dept;
+----------+-------------+------+-----+---------+----------------+
| Field | Type  | Null | Key | Default | Extra   |
+----------+-------------+------+-----+---------+----------------+
| id  | int(11)  | NO | PRI | NULL | auto_increment |
| deptName | varchar(30) | YES | MUL | NULL |    |
| locAdd | varchar(40) | YES |  | NULL |    |
+----------+-------------+------+-----+---------+----------------+
3 rows in set (0.01 sec)

mysql> select * from tbl_emp a where a.deptld in (select id from tbl_dept);

mysql> select * from tbl_emp a where a.deptld=1 or a.deptld=2 or a.deptld=3

mysql> select * from tbl_emp a where exists (select 1 from tbl_dept b where a.deptld = b.id );

for (i = 0; i < count(A); i++) { //遍历A的总记录数
　　a = get_record(A, i); //从A表逐条获取记录
　　if (B.id = a[id]) //如果子条件成立
　　　　result[] = a;
}
return result;

mysql> select * from tbl_emp a where a.deptld in (select id from tbl_dept);

mysql> select * from tbl_emp a where exists (select 1 from tbl_dept b where a.deptld = b.id );

SELECT * FROM A WHERE id IN (SELECT id FROM B);

SELECT * FROM a WHERE EXISTS(SELECT 1 FROM b WHERE B.id = A.id);

SELECT * FROM A;
SELECT I FROM B WHERE B.id = A.id;