数据库的各种连接查询你是否真了解

下面就让我们来做个小测试，一一探查常用数据库的连接原理。

--数据测试环境准备：

SQL> create table t1(a int);

Table created

SQL> create table t2(b int);

Table created

SQL> insert into t1 values(1);

1 row inserted

SQL> insert into t1 values(2);

1 row inserted

SQL> insert into t1 values(3);

1 row inserted

SQL> insert into t2 values(1);

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1 row inserted

SQL> insert into t2 values(2);

1 row inserted

SQL> insert into t2 values(4);

1 row inserted

SQL> commit;

Commit complete

--内连接，符合条件的行列出，可以看作是两个表的交集

SQL> select t1.*,t2.* from t1 inner join t2 on t1.a=t2.b;

A B

--------------------------------------- ---------------------------------------

1 1

2 2

--交叉连接，即返回两个表的笛卡尔积

SQL> select t1.*,t2.* from t1 cross join t2;

A B

--------------------------------------- ---------------------------------------

1 1

1 2