009、SQL数据库 表单 数据---条件查询

--查询年龄大于20岁
SELECT * FROM student WHERE age > 20;

--查询年龄不小于20岁的
SELECT * FROM student WHERE age >= 20;

--查询年龄等于20岁的,注意是单引号
SELECT * FROM student WHERE age = 20;

--查询年龄不等20岁的
SELECT * FROM student WHERE age != 20;
SELECT * FROM student WHERE age <> 20;

--查询年龄在20~30岁之间的
SELECT * FROM student WHERE age >20 && age <20; #不推荐
SELECT * FROM student WHERE age>20 AND age<20;  #不推荐
SELECT * FROM student WHERE age BETWEEN 20 AND 30;  #推荐

--查询年龄为18、20、22
SELECT * FROM student WHERE age = 18 || age = 20 || age =22; #不推荐
SELECT * FROM student WHERE age = 18 OR age == 20 OR age =22; #不推荐
SELECT * FROM student WHERE age IN(18,20,22) #推荐

--查询数学考试成绩为null的，注意对于null的判断不是用=好，而是用IS
SELECT * FROM student WHERE math IS NULL; 

--查询数学考试成绩部位null的
SELECT * FROM student WHERE age IS NOT NULL;

--模糊查询

    占位符：_表示单个字符;%表示多个字符

#查询姓马的所有人
SELECT * FROM student WHERE NAME LIKE '马%';

#查询名字中第二字为国的所有人
SELECT * FROM student WHERE NAME LIKE '_国%';

#查询名字个数为3个字的所有人
SELECT * FROM student WHERE NAME LIKE '___';

#查询名字中有国的所有人
SELECT * FROM student WHERE NAME LIKE '%国%';