mysql语法备忘

1. 存储过程和游标
   核心： 
   

        #变量必须声明在最前面
        declare noMoreRows bool default false;
        declare _id int;
        declare _name varchar(25);
        
        #声明游标
        declare xxRows cursor for select * from xx;
        #当游标移动到最后空行时设置noMoreRows为true
        declare CONTINUE handler for NOT FOUND set noMoreRows=true;
        
        #打开游标
        open xxRows;
        
        #创建临时内存表
        drop table if exists xxTem;
        create temporary table xxTem
        (id int,name varchar(25), primary key(id))
        engine=memory;
        truncate table xxTem;
        
        #遍历游标
        repeat
            #取出数据到变量
            fetch xxRows into _id,_name;     
           #过滤掉重复行，将游标取出结果插入到临时表        
            if not exists(select 1 from xxTem where id=_id) then
                insert into xxTem(id,name) values(_id,_name);
            end if;         
        until noMoreRows=1
        end repeat;
        
        #关闭游标
        close xxRows;  
        #根据参数裁剪临时表结果返回
        select * from xxTem limit `skip`,`count`;

   完整： 
   

   CREATE TABLE `xx` (
       `id` INT(11) NOT NULL,
       `name` VARCHAR(45) NULL DEFAULT NULL,
       PRIMARY KEY (`id`)
   )
   COLLATE='utf8_general_ci'
   ENGINE=InnoDB;
   
   CREATE DEFINER=`root`@`localhost` PROCEDURE `proc1`(IN `skip` int, IN `count` int)
       LANGUAGE SQL
       NOT DETERMINISTIC
       CONTAINS SQL
       SQL SECURITY DEFINER
       COMMENT ''
   BEGIN    
        #变量必须声明在最前面
        declare noMoreRows bool default false;
        declare _id int;
        declare _name varchar(25);
        
        #声明游标
        declare xxRows cursor for select * from xx;
        #当游标移动到最后空行时设置noMoreRows为true
        declare CONTINUE handler for NOT FOUND set noMoreRows=true;
        
        #打开游标
        open xxRows;
        
        #创建临时内存表
        drop table if exists xxTem;
        create temporary table xxTem
        (id int,name varchar(25), primary key(id))
        engine=memory;
        truncate table xxTem;
        
        #遍历游标
        repeat
            #取出数据到变量
            fetch xxRows into _id,_name;     
           #过滤掉重复行，将游标取出结果插入到临时表        
            if not exists(select 1 from xxTem where id=_id) then
                insert into xxTem(id,name) values(_id,_name);
            end if;         
        until noMoreRows=1
        end repeat;
        
        #关闭游标
        close xxRows;  
        #根据参数裁剪临时表结果返回
        select * from xxTem limit `skip`,`count`;
   END

2. 动态sql：随机获取某表的若干行
   下面是存储过程的代码。过程包含三个参数，分别是表名，行数和主键id。过程只支持int类型单主键的表。主键可以省略为空，这样过程会自动查找主键，不过这样比较慢。
   

   CREATE DEFINER=`root`@`%` PROCEDURE `rand_data`(IN `tbName` VARCHAR(50), IN `rowCnt` INT, IN `tbKey` VARCHAR(50))
       LANGUAGE SQL
       NOT DETERMINISTIC
       CONTAINS SQL
       SQL SECURITY DEFINER
       COMMENT '随机获取若干记录,只适用于单主键表'
   BEGIN
   
       #获取主键名
       IF tbKey IS NOT NULL THEN
           SET @tbKey=tbKey;#参数里面已经有，这种情况比较快
       ELSE
           #参数里面没，从系统表查找主键，比较耗时
           SELECT @tbKey:=c.COLUMN_NAME
           FROM
                 INFORMATION_SCHEMA.TABLE_CONSTRAINTS AS t,
                 INFORMATION_SCHEMA.KEY_COLUMN_USAGE AS c
           WHERE
                 t.TABLE_NAME = c.TABLE_NAME
                 AND t.TABLE_SCHEMA = c.TABLE_SCHEMA
               AND t.TABLE_SCHEMA = database()
               AND t.TABLE_NAME = tbName
               AND t.CONSTRAINT_TYPE = 'PRIMARY KEY';
       END IF;
           
       #获取最大id,最小id和记录数
       SET @getMaxIdSql = CONCAT('SELECT @maxId:=MAX(', @tbKey, '),',
           '@minId:=MIN(', @tbKey, '),',
           '@totalCnt:=COUNT(', @tbKey, ')',
           ' FROM `', tbName, '`;');
       PREPARE getMaxId FROM @getMaxIdSql;
       EXECUTE getMaxId;
       DEALLOCATE PREPARE getMaxId;
       
       #创建临时表
       DROP TABLE IF EXISTS rand_tt;
       SET @temTbSql = CONCAT('CREATE TEMPORARY TABLE rand_tt SELECT 0 aid,tb.* FROM `',tbName,'` tb LIMIT 0;');
       PREPARE temTb FROM @temTbSql;
       EXECUTE temTb;
       DEALLOCATE PREPARE temTb;
       
       #构建获取一条记录的sql
       SET @randRowSql = CONCAT('INSERT INTO rand_tt SELECT @cnt:=@cnt+1 aid,tb.* FROM ',
           tbName, ' tb WHERE tb.', @tbKey, '=?;');
       PREPARE addRow FROM @randRowSql;
       
       #生成随机记录
       SET @cnt=0;
       insertLoop: LOOP
           SET @id=FLOOR(RAND()*(@maxId-@minId)+@minId);
           IF NOT EXISTS (SELECT id FROM rand_tt WHERE id=@id) THEN
               EXECUTE addRow USING @id;
               IF @cnt >= rowCnt OR @cnt >= @totalCnt THEN
                   LEAVE insertLoop;
               END IF;
           END IF;
       END LOOP insertLoop;    
       DEALLOCATE PREPARE addRow;
       
       #返回数据
       ALTER TABLE rand_tt DROP COLUMN aid;
       SELECT * FROM rand_tt;
   END

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