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','ZHOU@163.com',2,'1988-09-09');

INSERT INTO tb_emp(NAME,sex,age,address,email,dept_id,diredate)
VALUES('CAI','女',30,'香港','CAI@163.com',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