1.MySQL的主要函數

1.1 聚合函數

group_concat()函數的使用

CREATE DATABASE bianbian4; USE bianbian4; CREATE TABLE emp (emp_id INT PRIMARY KEY auto_increment COMMENT '編號',emp_name VARCHAR ( 20 ) NOT NULL DEFAULT '' COMMENT '姓名',salary DECIMAL ( 10, 2 ) NOT NULL DEFAULT 0 COMMENT '工資',department VARCHAR ( 20 ) NOT NULL DEFAULT '' COMMENT '部門' ); INSERT INTO emp ( emp_name, salary, department ) VALUES( '張晶晶', 5000, '財務部' ),( '王飛飛', 5800, '財務部' ),( '趙剛', 6200, '財務部' ),( '劉小貝', 5700, '人事部' ),( '王大鵬', 6700, '人事部' ),( '張小裴', 5200, '人事部' ),( '劉云云', 7500, '銷售部' ),( '劉云鵬', 7200, '銷售部' ),( '劉云鵬', 7800, '銷售部' );-- 將所有員工的名字合并成一行 SELECT GROUP_CONCAT(emp_name) FROM emp; -- 指定分隔符合并 SELECT GROUP_CONCAT(emp_name SEPARATOR ';') FROM emp; -- 指定排序方式和分隔符 SELECT department,GROUP_CONCAT(emp_name ORDER BY salary desc SEPARATOR ';') FROM emp GROUP BY department;

1.2 數學函數

-- 數學函數 SELECT abs(-10); SELECT abs(10); SELECT abs(表達式或者字段） FROM 表; -- ceil 向上取證; select ceil(1.1); -- 2 select ceil(1.0); -- 1 -- 向下取整 select floor(1.1); -- 1 select floor(1.9); -- 1 -- 取最大值 select greatest(1,2); -- 2 -- 最小值 select least(1,2,3); -- 1

1.3 字符串函數

1.4 日期函數

1.5 控制流函數

1.5.1 if邏輯判斷語句

判斷狀態時，可以使用IF函數進行判斷：

1.5.2 case when函數

case when 相當于 C語言的switch()語句

1.6 窗口函數

窗口函數分類：

窗口函數語法結構：

1.6.1 序號函數

序號函數：row_number()、rank()、dense_rank()，可以用來實現分組排序，并添加序號。

use bianbian4; create table employee( dname varchar(20), eid varchar(20), ename varchar(20), hiredate date, salary double ); -- 數據類容 insert into employee values ('研發部','1001','劉備','2021-11-01',3000), ('研發部','1002','關羽','2021-11-02',5000), ('研發部','1003','張飛','2021-11-03',7000), ('研發部','1004','趙云','2021-11-04',7000), ('研發部','1005','馬超','2021-11-05',4000), ('研發部','1006','黃忠','2021-11-06',4000),('銷售部','1001','曹操','2021-11-01',2000), ('銷售部','1002','許褚','2021-11-02',3000), ('銷售部','1003','典韋','2021-11-03',5000), ('銷售部','1004','張遼','2021-11-04',6000), ('銷售部','1005','徐晃','2021-11-05',9000), ('銷售部','1006','曹洪','2021-11-06',6000); -- 對每個部門的員工按照薪資排序，并給出排名 select dname,ename,salary, row_number() over (partition by dname order by salary desc) as row_rank, rank() over (partition by dname order by salary desc) as rk, dense_rank() over (partition by dname order by salary desc) as dense_rk from employee;-- 求出每個部門薪資排在前三的員工，-分組求topN select * from ( select dname,ename,salary, dense_rank() over (partition by dname order by salary desc) as dense_rk from employee ) as t where t.dense_rk<=3; -- 對所有員工進行全局排序 select dname,ename,salary, dense_rank() over (order by salary desc) as dense_rk from employee

row_number()：不考慮相不相同，順序的數字不變，1，2，3
rank()：考慮相同的情況，跳數字
dense_rank()：考慮相同情況， 不跳數字
如下圖：

1.6.2 開窗聚合函數

-- 按照分組日期累加 select dname,ename,hiredate,salary, sum(salary) over (partition by dname order by hiredate ) as c1 from employee

-- 如果沒有order by 將分組內的數據全部相加 select dname,ename,hiredate,salary, sum(salary) over (partition by dname ) as c1 from employee

實現區間統計，以sum為例子，數據是上面創建的數據：

-- 指定范圍進行相加 -- 起始行： unbounded preceding 到當前的行 current row select dname,ename,hiredate,salary, sum(salary) over (partition by dname order by hiredate rows between unbounded preceding and current row) as c1 from employee-- 當前行的上面三行 select dname,ename,hiredate,salary, sum(salary) over (partition by dname order by hiredate rows between 3 preceding and current row) as c1 from employee-- 向上三行+當前行+下面一行 select dname,ename,hiredate,salary, sum(salary) over (partition by dname order by hiredate rows between 3 preceding and 1 following) as c1 from employee-- 當前行到最后 select dname,ename,hiredate,salary, sum(salary) over (partition by dname order by hiredate rows between current row and unbounded following) as c1 from employee

1.6.3 分布函數

cume_dist()的介紹：
用途：分組內小于、等于當前rank值的行數/分組內總行數
應用場景：查詢小于等于當前薪資的比列

cume_dist()的實例：

use bianbian4; selectdname,ename,salary,cume_dist() over(order by salary) as rn1,cume_dist() over(partition by dname order by salary) as rn2 from employee; /* 不加partition by 是全部行數： rn1 表示整個工資比當前數小于等于的數所占比例 3/12 = 0.25 (3000) 5/12 = 0.4166666666666667 (4000)有partition by,看本組： rn2表示含義案例： 1 / 6 = 0.16666666666666666 (3000,在研發組中） */

cume_dist()的結果：

percent_rank的介紹：

percent_rank代碼：

selectdname,ename,salary,rank() over(partition by dname order by salary desc) as rn1,percent_rank() over(partition by dname order by salary desc) as rn2 from employee; /* rn2: 第一行：（1-1）/ (6-1) = 0 第二行：（1-1）/ (6-1) = 0 第三行：（3-1）/ (6-1) = 0.4 */

percent_rank()結果：

1.6.4 前后函數 LAG和LEAD

selectdname,ename,salary,hiredate,-- 將上一行的，放在當前的后面，如果沒有默認'2000-01-01'lag(hiredate,1,'2000-01-01') over(partition by dname order by hiredate ) as rn1,-- 將上兩行的值，放在當前的后面，如果沒有默認nulllag(hiredate,2) over(partition by dname order by hiredate ) as rn2 from employee;

1.6.5 頭尾函數first_value和last_value

first_value()和last_value()案例：

selectdname,ename,salary,hiredate,-- 到目前為止的第一個first_value(salary) over(partition by dname order by hiredate ) as first_,-- 到目前為止的最后一個last_value(salary) over(partition by dname order by hiredate ) as last_ from employee;

1.6.6 其他函數nth_value(expr,n)、ntile(n)

nth_value(expr,n)案例：

selectdname,ename,salary,hiredate,-- 到目前為止的排第1行的薪資nth_value(salary,1) over(partition by dname order by hiredate ) as first_salary,-- 到目前為止的排第2行的薪資nth_value(salary,2) over(partition by dname order by hiredate ) as second_salary from employee;

ntile案例：

selectdname,ename,salary,hiredate,-- 分成3組ntile(3) over(partition by dname order by hiredate ) as nt from employee; -- 取出每一個部門第一組員工 select * from ( selectdname,ename,salary,hiredate,-- 分成3組ntile(3) over(partition by dname order by hiredate ) as nt from employee ) t where t.nt =1;

參考

https://www.bilibili.com/video/BV1iF411z7Pu?p=99&spm_id_from=pageDriver

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