本文章目的：力求清晰明了讲解SQL语句的内连接的各种应用，没有深奥的理解！

前奏：这篇文章和下篇文章会将内连接和外连接讲解清楚
SQL语句的多表查询常用的有以下几种:
两表联合查询
（1）内连接
（2）外连接（分左外连接、右外连接）
（3）全外连接
（4）自连接
三表查询
（1）三表查询
本片讲解两表联合查询的内连接：
第一步：准备表，员工表emp，部门表dept
关联关系：员工表的“deptno”字段关联部门表的“deptno”

建表语句如下：

create table DEPT
(
  DEPTNO int(2) not null,
  DNAME  VARCHAR(14),
  LOC    VARCHAR(13)
);
alter table DEPT
  add constraint PK_DEPT primary key (DEPTNO);
create table EMP
(
  EMPNO    int(4) primary key,
  ENAME    VARCHAR(10),
  JOB      VARCHAR(9),
  MGR      int(4),
  HIREDATE DATE,
  SAL      double(7,2),
  COMM     double(7,2),
  DEPTNO   int(2)
);
alter table EMP
  add constraint FK_DEPTNO foreign key (DEPTNO)
  references DEPT (DEPTNO);

表结构如下：

插入数据：

insert into DEPT (DEPTNO, DNAME, LOC)values (10, 'ACCOUNTING', 'NEW YORK');
insert into DEPT (DEPTNO, DNAME, LOC)values (20, 'RESEARCH', 'DALLAS');
insert into DEPT (DEPTNO, DNAME, LOC)values (30, 'SALES', 'CHICAGO');
insert into DEPT (DEPTNO, DNAME, LOC)values (40, 'OPERATIONS', 'BOSTON');

insert into DEPT (DEPTNO, DNAME, LOC)values (10, 'ACCOUNTING', 'NEW YORK');
insert into DEPT (DEPTNO, DNAME, LOC)values (20, 'RESEARCH', 'DALLAS');
insert into DEPT (DEPTNO, DNAME, LOC)values (30, 'SALES', 'CHICAGO');
insert into DEPT (DEPTNO, DNAME, LOC)values (40, 'OPERATIONS', 'BOSTON');
commit;
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7369, 'SMITH', 'CLERK', 7902, '1980-12-17', 800, null, 20);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7499, 'ALLEN', 'SALESMAN', 7698, '1981-02-20', 1600, 300, 30);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7521, 'WARD', 'SALESMAN', 7698, '1981-02-22', 1250, 500, 30);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7566, 'JONES', 'MANAGER', 7839, '1981-04-02', 2975, null, 20);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7654, 'MARTIN', 'SALESMAN', 7698, '1981-09-28', 1250, 1400, 30);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7698, 'BLAKE', 'MANAGER', 7839, '1981-05-01', 2850, null, 30);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7782, 'CLARK', 'MANAGER', 7839, '1981-06-09', 2450, null, 10);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7788, 'SCOTT', 'ANALYST', 7566, '1987-04-19', 3000, null, 20);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7839, 'KING', 'PRESIDENT', null, '1981-11-17', 5000, null, 10);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7844, 'TURNER', 'SALESMAN', 7698, '1981-09-08', 1500, 0, 30);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7876, 'ADAMS', 'CLERK', 7788, '1987-05-23', 1100, null, 20);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7900, 'JAMES', 'CLERK', 7698, '1981-12-03', 950, null, 30);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7902, 'FORD', 'ANALYST', 7566, '1981-12-03', 3000, null, 20);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7934, 'MILLER', 'CLERK', 7782, '1982-01-23', 1300, null, 10);

第二步：讲解内连接

（1）笛卡尔积现象：（什么是笛卡尔积，在此不在讲解，如果有需要，可以留言，我再补充）

通过关键字：CROSS JOIN

优点：有助于理解多表查询

缺点：交叉连接会产生笛卡尔积，产生多条无用结果

解决方案：（1