自然连接（NATURAL JOIN）是一种特殊的等值连接，将表中具有相同名称的列自动进行匹配。
1.自然连接不必指定任何连接条件。
SQL> desc emp
Name Null? Type
----------------- -------- ------------------
EMPNO NOT NULL NUMBER(4)
ENAME VARCHAR2(10)
JOB VARCHAR2(9)
MGR NUMBER(4)
HIREDATE DATE
SAL NUMBER(7,2)
COMM NUMBER(7,2)
DEPTNO NUMBER(2)
SQL> desc dept
Name Null? Type
------------------ -------- ------------------
DEPTNO NOT NULL NUMBER(2)
DNAME VARCHAR2(14)
LOC VARCHAR2(13)
以上两张表中，有deptno这列名字相同，而且数据类型一致。
SQL> select *
2 from emp natural join dept;
DEPTNO EMPNO ENAME JOB MGR HIREDATE SAL COMM DNAME LOC
---------- ---------- ---------- ---------- ---------- ------------ ---------- ---------- ---------- ---------
10 7782 CLARK MANAGER 7839 09-JUN-81 2450 ACCOUNTING NEW YORK
10 7839 KING PRESIDENT 17-NOV-81 5000 ACCOUNTING NEW YORK
10 7934 MILLER CLERK 7782 23-JAN-82 1300 ACCOUNTING NEW YORK
20 7566 JONES MANAGER 7839 02-APR-81 2975 RESEARCH DALLAS
20 7902 FORD ANALYST 7566 03-DEC-81 3000 RESEARCH DALLAS
20 7876 ADAMS CLERK 7788 23-MAY-87 1100 RESEARCH DALLAS
20 7369 SMITH CLERK 7902 17-DEC-80 800 RESEARCH DALLAS
20 7788 SCOTT ANALYST 7566 19-APR-87 3000 RESEARCH DALLAS
30 7521 WARD SALESMAN 7698 22-FEB-81 1250 500 SALES CHICAGO
30 7844 TURNER SALESMAN 7698 08-SEP-81 1500 0 SALES CHICAGO
30 7499 ALLEN SALESMAN 7698 20-FEB-81 1600 300 SALES CHICAGO
30 7900 JAMES CLERK 7698 03-DEC-81 950 SALES CHICAGO
30 7698 BLAKE MANAGER 7839 01-MAY-81 2850 SALES CHICAGO
30 7654 MARTIN SALESMAN 7698 28-SEP-81 1250 1400 SALES CHICAGO
14 rows selected.
自然连接之后，两张表等值连接，DEPTNO这列自动合并成一列。
2.如果两张表中有相同名字的列，但是数据类型不一致，如果能隐式转换，则能正常连接，但如果隐式转换不成功，则报错。
SQL> create table emp2 as select * from emp;
Table created.
SQL> create table dept2 as select * from dept;
Table created.
SQL> alter table emp2 modify deptno number(10);
Table altered.
SQL> select * from emp2 natural join dept2;
DEPTNO EMPNO ENAME JOB MGR HIREDATE SAL COMM DNAME LOC
---------- ---------- ---------- ---------- ---------- ------------ ---------- ---------- ---------- ---------
20 7369 SMITH CLERK 7902 17-DEC-80 800 RESEARCH DALLAS
30 7499 ALLEN SALESMAN 7698 20-FEB-81 1600 300 SALES CHICAGO
30 7521 WARD SALESMAN 7698 22-FEB-81 1250 500 SALES CHICAGO
20 7566 JONES MANAGER 7839 02-APR-81 2975 RESEARCH DALLAS
30 7654 MARTIN
