用户自定义函数（user-defined function,UDF）。UDF可以没有参数,但UDF必须有且只有一个返回值

创建UDF

CREATE FUNCTION function_name(parameter_name type,[parameter_name2 type2,...])
RETURNS  {STRING|INTEGER|REAL|DECIMAL}
BEGIN
    body
END

删除UDF

DROP FUNCTION fun_name 删除自定义函数

DELIMITER “字符串”

更改结束标识

mysql> DELIMITER "//"
mysql> SHOW TABLES //
+----------------+
| Tables_in_test |
+----------------+
| provinces      |
| user           |
| user2          |
| userinfo       |
+----------------+

实例

// 计算两个数的平均值
mysql> CREATE FUNCTION f2_2(num1 SMALLINT,num2 SMALLINT)
    -> RETURNS FLOAT(10,2)
    -> RETURN (num1+num2)/2;

mysql> CREATE FUNCTION adduser(username VARCHAR(20))
    -> RETURNS VARCHAR(20)
    -> BEGIN
    -> INSERT INTO user(username) VALUES(username);
    -> RETURN LAST_INSERT_ID();
    -> END
    -> //

定义局部变量

DECLARE var_name [,var_name…] date_type [DEFAULT VALUE];

为变量赋值语法(全局变量也是如此):
- SET parameter_name = value[,parameter_name = value…]
- SELECT INTO parameter_name

# 使用default 指定默认值
CREATE FUNCTION add(num INT)
RETURNS INT
BEGIN
    DECLARE temp INT DEFAULT 10 ; 
    RETURN temp+num;
END

# 使用select into 为变量赋值
CREATE FUNCTION userNum()
RETURNS INT
BEGIN
    DECLARE temp INT DEFAULT 10 ; 
    SELECT COUNT(id) FROM user INTO temp;
    RETURN temp;
END

用户变量(可以理解成全局变量)

• 声名：SET @param_name = value；
• 使用：SELECT @allParam;

# 声名A=100
mysql> SET @A = 100 ;
#声名B= 200
mysql> SET @B = 200;
#声名C=A+B
mysql> SET @C = @A+@B;
#输出C
mysql> SELECT @C ;
+------+
| @C   |
+------+
|  300 |
+------+

判断语句 IF

IF condition THEN statement
[ELSEIF condition THEN statement]
[ELSE statement]
END IF

CREATE FUNCTION if_test(age INT)
RETURNS VARCHAR(20)
BEGIN
IF age>18 THEN RETURN "成年";
ELSEIF age>10 THEN RETURN "未成年";
ELSE RETURN "小学生";
END IF;
END

分支语句 CASE

CASE case_value
WHEN when_value THEN statement
[WHEN when_value THEN statement] …
[ELSE statement_list]
END CASE

另一种方式：
CASE
WHEN search_condition THEN statement_list
[WHEN search_condition THEN statement_list] …
[ELSE statement_list]
END CASE

# 第一种方式
CREATE FUNCTION case_test(age INT)
RETURNS VARCHAR(20)
BEGIN
CASE age
WHEN 20 THEN RETURN "20#";
WHEN 30 THEN RETURN "30#";
ELSE RETURN "#";
END CASE;
END

# 第二种方式
CREATE FUNCTION case_test2(age INT)
RETURNS VARCHAR(20)
BEGIN
CASE 
WHEN age=20 THEN RETURN "20#";
WHEN age=30 THEN RETURN "30#";
ELSE RETURN "#";
END CASE;
END

循环 LOOP

[begin_label:] LOOP
statement
END LOOP [end_label] ;

LEAVE

leave用于终止loop循环
LEAVE label

# 累加
CREATE FUNCTION loop_test()
RETURNS INT 
BEGIN
    DECLARE count int DEFAULT 10;
    adds: LOOP
        SET count = count+1;
        IF count > 100 THEN LEAVE adds;
        END IF;
    END LOOP adds ;
    RETURN count;
END

关于循环还有许多语句，以后用到再做总结