MySQL学习笔记——函数

常用函数

ALTER TABLE tb_emp

ADD diredate VARCHAR(20);



#插入数据

INSERT INTO tb_dept()

VALUE(4,'市场部','负责市场工作');



# concat 连接

SELECT CONCAT(NAME,sex) FROM tb_emp;



# UPPER 转换大写

SELECT UPPER(NAME) FROM tb_emp WHERE dept_id=1;



# 返回字符串长度

SELECT LENGTH(NAME) FROM tb_emp WHERE dept_id=1;



# 返回部分字符

SELECT SUBSTR(NAME,2,2) FROM tb_emp WHERE dept_id=1;



# 返回当前

SELECT NOW();



# 查询时间是1981年

SELECT * FROM tb_emp

WHERE YEAR(diredate) = 1981

AND MONTH(diredate) = 1982;



#插入时间

INSERT INTO tb_emp(NAME,sex,age,address,email,dept_id,diredate)

VALUES('ZHOU','男',33,'香港','[email protected]',2,'1988-09-09');



INSERT INTO tb_emp(NAME,sex,age,address,email,dept_id,diredate)

VALUES('CAI','女',30,'香港','[email protected]',2,NOW());



# 条件判断语句

SELECT NAME,sex,age '原来年龄'

    CASE 

    WHEN age IS NULL THEN 100

    ELSE age

    END AS '年龄'

FROM tb_emp;



# IFNULL函数    如果字段不为NULL，则取第二个值，如果为空，择取第三个值

SELECT NAME,IFNULL(age,age+100,100) AS age2 FROM tb_emp;



# IFNULL函数    如果字段不为NULL，则直接去该值，如果为空，择取第二个值

SELECT NAME,IFNULL(age,100) AS age2 FROM tb_emp;

 

 

聚合函数

 

# 聚合函数，也叫组合函数，忽略空值

SELECT AVG(age) FROM tb_emp;



SELECT SUM(age) FROM tb_emp;



SELECT MAX(age) FROM tb_emp;



SELECT MIN(age) FROM tb_emp;



SELECT AVG(age) AS '平均年龄',SUM(age) AS '总年龄',MAX(age) AS '最高年龄',MIN(age) AS '最低年龄'

FROM tb_emp WHERE dept_id=1;



# COUNT不统计null，统计的是行数/记录数

SELECT COUNT(*) FROM tb_emp

SELECT COUNT(email) FROM tb_emp



# 不统计重复记录

SELECT COUNT(DISTINCT diredate) FROM tb_emp



# 分组统计 GROUP BY

# 每个部门的平均年龄

SELECT dept_id,AVG(age) FROM tb_emp GROUP BY dept_id



SELECT dept_id,AVG(age),address FROM tb_emp GROUP BY dept_id,address



# 限定查询结果 HAVING 不能使用where,where子句中不可以使用函数

SELECT dept_id,AVG(age) FROM tb_emp GROUP BY dept_id

HAVING AVG(age)>23

ORDER BY AVG(age) DESC;



# LIMIT 常用来分页

SELECT * FROM tb_emp LIMIT 5; #查询前5个记录

SELECT * FROM tb_emp LIMIT 5,10; #查询前6-10个记录