where 约束

where字句中可以使用：

1. 比较运算符：> < >= <= <> !=
2. between 80 and 100 值在10到20之间
3. in(80,90,100) 值是10或20或30
4. like 'egon%'
    pattern可以是%或_，
    %表示任意多字符
    _表示一个字符
5. 逻辑运算符：在多个条件直接可以使用逻辑运算符 and or not

 

 

#1:单条件查询
    SELECT name FROM employee
        WHERE post='sale';
        
#2:多条件查询
    SELECT name,salary FROM employee
        WHERE post='teacher' AND salary>10000;

#3:关键字BETWEEN AND
    SELECT name,salary FROM employee
        WHERE salary BETWEEN 10000 AND 20000;

    SELECT name,salary FROM employee
        WHERE salary NOT BETWEEN 10000 AND 20000;
    
#4:关键字IS NULL(判断某个字段是否为NULL不能用等号，需要用IS)
    SELECT name,post_comment FROM employee
        WHERE post_comment IS NULL;

    SELECT name,post_comment FROM employee
        WHERE post_comment IS NOT NULL;
        
    SELECT name,post_comment FROM employee
        WHERE post_comment=''; 注意''是空字符串，不是null
    ps：
        执行
        update employee set post_comment='' where id=2;
        再用上条查看，就会有结果了

#5:关键字IN集合查询
    SELECT name,salary FROM employee
        WHERE salary=3000 OR salary=3500 OR salary=4000 OR salary=9000 ;
    
    SELECT name,salary FROM employee
        WHERE salary IN (3000,3500,4000,9000) ;

    SELECT name,salary FROM employee
        WHERE salary NOT IN (3000,3500,4000,9000) ;

#6:关键字LIKE模糊查询
    通配符’%’
    SELECT * FROM employee
            WHERE name LIKE 'eg%';

    通配符’_’
    SELECT * FROM employee
            WHERE name LIKE 'al__';