一、分支结构
1. if函数
语法：if(表达式1,表达式2,表达式3)
如果表达式1成立，则返回表达式2的值，否则返回表达式3的值
mysql> select if(2<3,'True','False');
+------------------------+
| if(2<3,'True','False') |
+------------------------+
| True |
+------------------------+
1 row in set (0.00 sec)
mysql> select if(2>3,'True','False');
+------------------------+
| if(2>3,'True','False') |
+------------------------+
| False |
+------------------------+
1 row in set (0.00 sec)
2. case结构
case 有两种用法：
① 语法：
case 表达式|变量|字段
when 判断的值1 then 返回值1
when 判断的值2 then 返回值2
...
when 判断的值n then 返回值n
else 返回值
end case
② 语法：
case
when 条件1 then 返回值1或语句1
when 条件2 then 返回值2或语句2
...
when 条件n then 返回值n或语句n
else 返回值或语句
end case
如果作为表达式使用，case结构可以在任何地方使用，如果作为独立的语句使用，则只能放在begin...end中使用。
例1：作为表达式使用
mysql> select name,case gender when 1 then '男' when 2 then '女' else '未知' end 性别 from students;
+-----------+--------+
| name | 性别 |
+-----------+--------+
| 李四 | 男 |
| 周芷若 | 女 |
| 赵敏 | 女 |
| Lucy | 女 |
| Tony | 男 |
| Lucy | 女 |
| 萧峰 | 男 |
+-----------+--------+
7 rows in set (0.00 sec)
例2：作为独立语句使用
mysql> delimiter $$
mysql> create procedure get_students_grade(in name varchar(20),in subject varchar(20))
-> begin
-> declare su_score int default 0;
-> select s.score into su_score from students st left join score s on st.id=s.student_id left join subject su on su.id=s.subject_id where st.name=name and su.name=subject;
-> case
-> when su_score >= 90 then select '优秀';
-> when su_score >= 80 then select '良好';
-> when su_score >= 60 then select '及格';
-> else select '不及格';
-> end case;
-> end $$
Query OK, 0 rows affected (0.11 sec)
mysql> delimiter ;
mysql> call get_students_grade('周芷若','数学');
+-----------+
| 不及格 |
+-----------+
| 不及格 |
+-----------+
1 row in set (0.00 sec)
3. if结构
语法：
if expr1 then 语句1
elseif expr2 then 语句2
...
[else 语句n]
end if;
if结构只能在begin...end中使用
mysql> delimiter $$
mysql> create procedure getGradre(in score int)
-> begin
-> if score >= 90 then select 'A';
-> elseif score >= 80 then select 'B';
-> elseif score >= 60 then select 'C';
-> else select 'D';
-> end if;
-> end $$
Query OK, 0 rows affected (0.02 sec)
mysql> delimiter ;
mysql> call getGradre(85);
+---+
| B |
+---+
| B |
+---+
1 row in set (0.00 sec)
二、循环结构
MySQL的循环结构分为while、loop、repeat三种，循环控制结构有：iterate（类似于continue）和leave（类似于break）。
1. while循环结构
语法：
[标签:]while 循环条件 do
循环体;
end while [标签];
2. loop循环结构
语法：
[标签:]loop
循环体;
end loop [标签];
可以用来模拟简单的死循环，搭配leave结束循环。
3. repeat循环结构
语法：
[标签:]repeat
循环体;
until 结束循环条件
end repeat [标签]
三、MySQL循环结构使用示例
1. 创建测试表
create table admin(
id int auto_increment primary key,
name varchar(20),
passwd char(32)
) engine=innodb default charset=utf8mb4 collate=utf8mb4_unicode_ci;
2. 创建存储过程，使用while循环结构，批量插入指定数量的数据
set autocommit = 0;#关闭事务的自动提交
delimiter $
create procedure insertAdmin(in insertCount int)
begin
declare i int default 1;
start transaction;
while insertCount >= i do
insert into admin(name,passwd) values(concat('abcd',i),md5('aaaa'));
set i = i+1;
end while;
commit;
end $
delimiter ;
call insertAdmin(2);
mysql> select * from admin;
+----+-------+----------------------------------+
| id | name | passwd |
+----+-------+----------------------------------+
| 1 | abcd1 | 74b87337454200d4d33f80c4663dc5e5 |
| 2 | abcd2 | 74b87337454200d4d33f80c4663dc5e5 |
+----+-------+----------------------------------+
2 rows in set (0.00 sec)
3. 创建存储过程，使用loop循环结构，批量插入指定数量的数据
delimiter $
create procedure insertAdminLoop(in insertCount int)
begin
declare i int default 1;
loopName:loop
insert into admin(name,passwd) values(concat('efgh',i),md5('bbbb'));
if insertCount <= i then leave loopName;
end if;
set i = i+1;
end loop loopName;
commit;
end $
delimiter ;
call insertAdminLoop(2);
4. 创建存储过程，使用repeat循环结构，批量插入指定数量的数据
mysql> delimiter $
mysql> create procedure insertAdminRepeat(in insertCount int)
-> begin
-> declare i int default 1;
-> repeat
-> insert into admin(name,passwd) values(concat('fwcweAX',i),md5(concat('sdfewf',i)));
-> set i = i+1;
-> until insertCount < i
-> end repeat;
-> commit;
-> end $
Query OK, 0 rows affected (0.01 sec)
mysql> delimiter ;
四、使用存储函数和存储过程灌入数据
1. 创建部门表
create table dept(
id int auto_increment primary key,
deptName varchar(30) default null,
address varchar(40) default null
)engine=innodb default charset=utf8mb4 collate=utf8mb4_unicode_ci;
2. 创建员工信息表
create table emp(
id int auto_increment primary key,
name varchar(20) default null,
age tinyint(3) default null,
deptid int default null,
empno int not null,
key idx_dept_id (deptid)
)engine=innodb default charset=utf8mb4 collate=utf8mb4_unicode_ci;
3. 创建函数，获取随机字符串
delimiter $
create function rand_str(n int) returns varchar(255) reads sql data
begin
declare chars_str varchar(60) default 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
declare return_str varchar(60) default '';
declare i int default 1;
while i <= n do
set return_str = concat(return_str,substr(chars_str,floor(rand()*52+1),1));
set i = i+1;
end while;
return return_str;
end $
4. 创建函数，获取部门随机编号
create function rand_number(from_num int,to_num int) returns int no sql
begin
declare i int default 0;
set i = floor(from_num + rand() * (to_num - from_num + 1));
return i;
end $
5. 创建存储过程，批量插入员工信息数据
create procedure insertEmp(in start_num int, in end_num int)
begin
declare i int default 0;
declare name_len int default 1;
set name_len = floor(5 + rand()*6);
set autocommit = 0;
start transaction;
repeat
set i = i+1;
insert into emp(name,age,deptid,empno) values(rand_str(name_len),rand_number(30,50),rand_number(1,100000),(start_num+i));
until end_num <= i
end repeat;
commit;
set autocommit=1;
end $
call insertEmp(100000,5000000);
6. 创建存储过程，批量插入部门信息数据
create procedure insertDept(in total int)
begin
declare i int default 0;
declare deptName_len int default 1;
declare addr_len int default 1;
set deptName_len = floor(6+rand()*7);
set addr_len = floor(10+rand()*11);
set autocommit = 0;
start transaction;
repeat
set i = i+ 1;
insert into dept(deptName,address,ceo) values(rand_str(deptName_len),rand_str(addr_len),rand_number(1,5000000));
until i>=total
end repeat;
commit;
set autocommit=1;
end $
call insertDept(100000);