条件查询
-- 条件查询 select 结果列 from 表名 where 条件
SELECT * FROM student WHERE height>=1.80;-- and 所有条件都要满足
SELECT * FROM student WHERE height<1.80 AND gender ='女';-- or 满足一个条件即可
SELECT * FROM student WHERE height=1.80 OR gender='女';
模糊查询
-- 模糊查询 like %字符% 通配符
SELECT NAME FROM student WHERE NAME LIKE '%杰';-- between 1.70 and 2.0== height>=1.70 and height<=2.0
SELECT NAME FROM student WHERE height BETWEEN 1.70 AND 2.0;
in/not in语句
-- 查询身高1.80,1,57,1,55
SELECT NAME FROM student WHERE height IN(1.80,1.57,1.55);SELECT NAME FROM student WHERE height NOT IN(1.80,1.57,1.55);
空值查询&联合查询
-- 查询值为空的数据
SELECT NAME FROM student WHERE height IS NULL;-- 查询值不为空的数据
SELECT NAME FROM student WHERE height IS NOT NULL;-- union 合并多个查询的结果 重复的会自动去除
SELECT NAME FROM student WHERE height=1.80
UNION
SELECT NAME FROM student WHERE gender='女'-- union all 合并多个查询的结果 重复的不会去除
SELECT NAME FROM student WHERE height=1.80
UNION ALL
SELECT NAME FROM student WHERE gender='男'
排序
-- 排序
-- asc 升序(默认) desc降序
SELECT * FROM student ORDER BY height;SELECT * FROM student ORDER BY height DESC;
有数量限制的查询
-- 数量限制 limit 开始位置,查询的数量
-- 实际使用场景,数据分页显示 一次只查询一部分数据,提高查询效率
SELECT * FROM student LIMIT 0,1;
SELECT * FROM student LIMIT 1,3;
分组查询
-- 分组查询
-- 将某类数据分到一个组中进行处理
-- group by 分组条件(列名)
SELECT COUNT(*),gender FROM student GROUP BY gender;
SELECT SUM(height) FROM student GROUP BY gender;
SELECT AVG(height) FROM student GROUP BY gender;
)