限制SELECT语句的返回行
语法：
使用WHERE关键字来限制数据行的返回，WHERE子句跟在FROM子句的后面。
SQL> select * from emp where deptno=10;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- ------------------- ---------- ---------- ----------
7782 CLARK MANAGER 7839 1981/06/09 00:00:00 2450 10
7839 KING PRESIDENT 1981/11/17 00:00:00 5000 10
7934 MILLER CLERK 7782 1982/01/23 00:00:00 1300 10当WHERE后面的表达式为true时，则返回行，否则跳过改行。
SQL> select * from emp where ename='KING';
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- ------------------- ---------- ---------- ----------
7839 KING PRESIDENT 1981/11/17 00:00:00 5000 10字符串是大小写敏感的
SQL> select * from emp where ename='king';
no rows selected条件表达式支持的比较操作符有如下
SQL> select * from emp where sal<2000;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- ------------------- ---------- ---------- ----------
7369 SMITH CLERK 7902 1980/12/17 00:00:00 800 20
7499 ALLEN SALESMAN 7698 1981/02/20 00:00:00 1600 300 30
7521 WARD SALESMAN 7698 1981/02/22 00:00:00 1250 500 30
7654 MARTIN SALESMAN 7698 1981/09/28 00:00:00 1250 1400 30
7844 TURNER SALESMAN 7698 1981/09/08 00:00:00 1500 0 30
7876 ADAMS CLERK 7788 1987/05/23 00:00:00 1100 20
7900 JAMES CLERK 7698 1981/12/03 00:00:00 950 30
7934 MILLER CLERK 7782 1982/01/23 00:00:00 1300 10BETWEEN AND 只要值在这个范围内，就返回值
SQL> select * from emp where sal between 1500 and 2000;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- ------------------- ---------- ---------- ----------
7499 ALLEN SALESMAN 7698 1981/02/20 00:00:00 1600 300 30
7844 TURNER SALESMAN 7698 1981/09/08 00:00:00 1500 0 30IN 只要列的值出现在列表中即返回行
SQL> SELECT * FROM EMP WHERE ENAME IN ('SMITH','KING');
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- ------------------- ---------- ---------- ----------
7369 SMITH CLERK 7902 1980/12/17 00:00:00 800 20
7839 KING PRESIDENT 1981/11/17 00:00:00 5000 10
LIKE 模糊查询
查询所有姓名以S开头的人
SQL> select * from emp where ename like 'S%';
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- ------------------- ---------- ---------- ----------
7369 SMITH CLERK 7902 1980/12/17 00:00:00 800 20
7788 SCOTT ANALYST 7566 1987/04/19 00:00:00 3000 20% 通配符，表示0或多个任意字符
查询所有姓名包含S的人
SQL> select * from emp where ename like '%S%';
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- ------------------- ---------- ---------- ----------
7369 SMITH CLERK 7902 1980/12/17 00:00:00 800 20
7566 JONES MANAGER 7839 1981/04/02 00:00:00 2975 20
7788 SCOTT ANALYST 7566 1987/04/19 00:00:00 3000 20
7876 ADAMS CLERK 7788 1987/05/23 00:00:00 1100 20
7900 JAMES CLERK 7698 1981/12/03 00:00:00 950 30_ 通配符表示任意一个字符
查询名字的第二个字符为O的人
SQL> select * from emp where ename like '_O%';
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- ------------------- ---------- ---------- ----------
7566 JONES MANAGER 7839 1981/04/02 00:00:00 2975 20
7902 FORD ANALYST 7566 1981/12/03 00:00:00 3000 20如果列的值本身就包含%或_字符呢？ 用escape关键字转义。
查询名字中含有_的人
SQL> select * from emp where ename like '%\_%' escape '\';
no rows selected过滤NULL值
过滤NULL值不能使用= <>来查询，null有一个专有的判断条件 IS NULL
SQL> SELECT * FROM EMP WHERE COMM IS NULL;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- ------------------- ---------- ---------- ----------
7369 SMITH CLERK 7902 1980/12/17 00:00:00 800 20
7566 JONES MANAGER 7839 1981/04/02 00:00:00 2975 20
7698 BLAKE MANAGER 7839 1981/05/01 00:00:00 2850 30
7782 CLARK MANAGER 7839 1981/06/09 00:00:00 2450 10
7788 SCOTT ANALYST 7566 1987/04/19 00:00:00 3000 20
7839 KING PRESIDENT 1981/11/17 00:00:00 5000 10
7876 ADAMS CLERK 7788 1987/05/23 00:00:00 1100 20
7900 JAMES CLERK 7698 1981/12/03 00:00:00 950 30
7902 FORD ANALYST 7566 1981/12/03 00:00:00 3000 20
7934 MILLER CLERK 7782 1982/01/23 00:00:00 1300 10
