MySQL变量、分支、循环、游标与条件处理程序全攻略

SHOW [GLOBAL | SESSION] VARIABLES [LIKE 'pattern'];

-- 查看所有全局变量
SHOW GLOBAL VARIABLES;
-- 查看所有会话变量（默认就是 SESSION）
SHOW SESSION VARIABLES;
SHOW VARIABLES;          -- 等价于上面
-- 模糊查询（LIKE 支持 % 通配符）
SHOW VARIABLES LIKE 'auto%';     -- 以 auto 开头
SHOW VARIABLES LIKE '%auto%';    -- 包含 auto
SHOW VARIABLES LIKE 'autocommit'; -- 精确匹配

SELECT @@[GLOBAL.|SESSION.]variable_name;

SELECT @@GLOBAL.autocommit;    -- 查看全局变量
SELECT @@SESSION.autocommit;   -- 查看会话变量
SELECT @@autocommit;           -- 默认查看会话变量

SET @@[GLOBAL.|SESSION.]variable_name = new_value;
-- 或
SET [GLOBAL | SESSION] variable_name = new_value;
-- 修改会话变量（当前会话生效）
SET @@SESSION.autocommit = 0;
SET autocommit = 0;            -- 默认修改会话级别
-- 修改全局变量（需要管理员权限，对之后新建的会话生效）
SET @@GLOBAL.autocommit = 0;
SET GLOBAL autocommit = 0;

SET @var_name = expr [, @var_name2 = expr2] ...;

SET @var_name := expr [, @var_name2 := expr2] ...;

SELECT @var_name := expr [, @var_name2 := expr2] ...;

SELECT 列名 INTO @var_name FROM 表名 WHERE ...;

-- 方式一
SET @var1 := (SELECT ename FROM test WHERE id = 1);
SELECT @var1;
-- 方式二
SELECT @var2 := (SELECT ename FROM test WHERE id = 1);
SELECT @var2;
-- 方式三
SELECT ename INTO @var3 FROM test WHERE id = 1;
SELECT @var3;

SELECT
    VARIABLE_NAME AS '变量名',
    VARIABLE_VALUE AS '变量值'
FROM performance_schema.user_variables_by_thread
WHERE THREAD_ID = (
    SELECT THREAD_ID
    FROM performance_schema.threads
    WHERE PROCESSLIST_ID = CONNECTION_ID()
);

DECLARE 变量名 变量类型 [DEFAULT 默认值] [, 变量名2 类型2 ...];

DELIMITER //
CREATE PROCEDURE P2()
BEGIN
    -- 必须在最开头声明
    DECLARE name VARCHAR(10) DEFAULT 'bksw';
    SELECT name;
    SELECT * FROM test WHERE ename = name;
END //
DELIMITER ;
CALL P2();

SET var_name = new_value;     -- 方式一
SET var_name := new_value;    -- 方式二（推荐）
SELECT 列名 INTO var_name FROM 表名 WHERE ...;  -- 方式三

BEGIN
    DECLARE name VARCHAR(10) DEFAULT 'bksw',
            age SMALLINT DEFAULT 18,
            gender CHAR(1) DEFAULT '男';
    SET name = 'hdsw';
    SET age := 20;
    SET gender = '女';
    SELECT name, age, gender;
END

IF 条件1 THEN
    statement_list
ELSEIF 条件2 THEN
    statement_list
ELSE
    statement_list
END IF;

DELIMITER //
CREATE PROCEDURE p_if(IN score INT)
BEGIN
    IF score >= 90 THEN
        SELECT '优秀' AS result;
    ELSEIF score >= 60 THEN
        SELECT '及格' AS result;
    ELSE
        SELECT '不及格' AS result;
    END IF;
END //
DELIMITER ;

CASE case_value
    WHEN val1 THEN statement_list
    WHEN val2 THEN statement_list
    ...
    ELSE statement_list
END CASE;

CASE
    WHEN expr1 THEN statement_list
    WHEN expr2 THEN statement_list
    ...
    ELSE statement_list
END CASE;

DELIMITER //
CREATE PROCEDURE p3()
BEGIN
    SET @var1 := 20;
    SET @var2 := 10;
    CASE @var1 + @var2
        WHEN 20 THEN SELECT 20;
        WHEN 10 THEN SELECT 10;
        WHEN 30 THEN SELECT 30;
        ELSE SELECT 'ERROR';
    END CASE;
END //
DELIMITER ;
CALL p3();

WHILE search_condition DO
    statement_list
END WHILE;

WHILE i < 10 DO
    SET i := i + 1;
END WHILE;

REPEAT
    statement_list
UNTIL search_condition
END REPEAT;

REPEAT
    SET i := i + 1;
UNTIL i >= 10
END REPEAT;

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

my_loop: LOOP
    SET i := i + 1;
    IF i >= 10 THEN
        LEAVE my_loop;       -- 退出循环
    END IF;
END LOOP my_loop;

my_loop: LOOP
    SET i := i + 1;
    IF i = 5 THEN
        ITERATE my_loop;     -- 跳过 i=5，直接进入下一次
    END IF;
    IF i >= 10 THEN
        LEAVE my_loop;        -- i>=10 时退出
    END IF;
    SELECT i;
END LOOP my_loop;

DECLARE 游标名 CURSOR FOR 查询语句;

OPEN 游标名;

FETCH 游标名 INTO 变量1[, 变量2, ...];

CLOSE 游标名;

DELIMITER //
DROP PROCEDURE IF EXISTS p4 //
CREATE PROCEDURE p4()
BEGIN
    DECLARE var1 SMALLINT DEFAULT 0;
    DECLARE var2 VARCHAR(10);
    DECLARE times TINYINT DEFAULT 3;
    DECLARE cursor_name CURSOR FOR SELECT * FROM test;
    OPEN cursor_name;
    WHILE times > 0 DO
        FETCH cursor_name INTO var1, var2;
        SELECT var1, var2;
        SET times := times - 1;
    END WHILE;
    CLOSE cursor_name;
END //
DELIMITER ;
CALL p4();

DECLARE handler_action HANDLER
    FOR condition_value [, condition_value] ...
    statement_list

-- 无限循环读取游标
WHILE TRUE DO
    FETCH cursor_name INTO var1, var2;
    SELECT var1, var2;
END WHILE;

ERROR 1329 (02000): No data - zero rows fetched, selected, or processed

DELIMITER //
DROP PROCEDURE IF EXISTS p4 //
CREATE PROCEDURE p4()
BEGIN
    DECLARE var1 SMALLINT DEFAULT 0;
    DECLARE var2 VARCHAR(10);
    DECLARE done BOOL DEFAULT TRUE;           -- 开关变量
    DECLARE cursor_name CURSOR FOR SELECT * FROM test;
    DECLARE CONTINUE HANDLER FOR NOT FOUND     -- 条件处理程序
        SET done := FALSE;
    OPEN cursor_name;
    WHILE done DO                               -- 用开关控制循环
        FETCH cursor_name INTO var1, var2;
        IF done THEN                            -- 防止最后一次fetch越界后还执行
            SELECT var1, var2;
        END IF;
    END WHILE;
    CLOSE cursor_name;
END //
DELIMITER ;
CALL p4();