第六课时 SQL WHERE 子句(2)
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SQL WHERE 子句(2)
请思考以下命令的返回结果:
SELECT * FROM emp WHERE empno=7900;
SELECT * FROM emp WHERE ename='SMITH';
SELECT * FROM emp
WHERE sal > 2000 and sal < 3000;
SELECT * FROM emp
WHERE sal > 2000 or sal < 500;
SELECT * FROM emp
WHERE not sal > 1500;
SELECT * FROM emp WHERE comm is mull;
SELECT * FROM emp
WHERE sal between 1500 and 3000;
SELECT * FROM emp
WHERE sal in (5000 , 3000 , 1500);
SELECT * FROM emp WHERE ename like ‘M%’;
SELECT * FROM emp WHERE ename like ‘%M%’;
SELECT * FROM emp WHERE ename like ‘%M_’;
WHERE 子句并不一定带比较运算符,当不带运算符时,会执行一个隐式转换。
SELECT studentNO FROM student WHERE 0;
则会返回一个空集,因为每一行记录 WHERE
都返回 false。
SELECT studentNO FROM student WHERE 1;
返回 student 表所有行中studentNO 列的值。
因为每一行记录WHERE 都返回 true。
逻辑运算优先级: ( ) not and or
动画演示:

操作视频演示:

