How does a primary table count the number of occurrences in the attached table?

How does a primary table count the number of occurrences in the attached table?

How does a primary table count the number of occurrences in the attached table? Create a master table: create table a1 (id int ,name char(20)); Create a schedule: create table a2 (tid int ,name char(20)); Insert data separately: mysql> insert into a1 values (1,'baidu');Query OK, 1 row affected (0.00 sec)mysql> insert into a1 values (2,'qq');Query OK, 1 row affected (0.00 sec)mysql> insert into a1 values (3,'sina');Query OK, 1 row affected (0.02 sec)mysql> insert into a2 values (1,'000');Query OK, 1 row affected (0.00 sec)mysql> insert into a2 values (1,'001');Query OK, 1 row affected (0.00 sec)mysql> insert into a2 values (1,'002');Query OK, 1 row affected (0.00 sec)mysql> insert into a2 values (1,'003');Query OK, 1 row affected (0.00 sec)mysql> insert into a2 values (2,'200');Query OK, 1 row affected (0.00 sec)mysql> insert into a2 values (2,'201');Query OK, 1 row affected (0.00 sec)mysql> insert into a2 values (2,'202');Query OK, 1 row affected (0.00 sec)mysql> insert into a2 values (2,'203');Query OK, 1 row affected (0.00 sec)mysql> insert into a2 values (2,'204');Query OK, 1 row affected (0.00 sec)mysql> insert into a2 values (2,'205');Query OK, 1 row affected (0.01 sec)mysql> insert into a2 values (3,'300');Query OK, 1 row affected (0.01 sec) SQL statement: select a.*,(select count(1) from a2 as b where b.tid=a.id) from a1 as a; In this way, you can query the number of IDs in table A1 !! Posted on2014-07-12 life learning, he Lai graduation reading (...) comment (...) Edit Blog homepage blog news flash programmer recruitment knowledge base