mysql 临时表

当查询大表时，我们可以通过构造临时表来避免频繁的遍历大表。临时表只有当前连接可见，当连接断开时会被自动drop，下文给出临时表的应用例子。

前提：

1、分别构造表t_demo_test1和t_demo_test2，并输入数据：

DROP TABLE IF EXISTS t_demo_test1;
CREATE TABLE IF NOT EXISTS t_demo_test1(
    tid int(20) NOT NULL AUTO_INCREMENT,
    userid int(20),
    day datetime NOT NULL,
    title varchar(50),
    content varchar(200),
    PRIMARY KEY(tid)
)ENGINE=InnoDB  DEFAULT CHARSET=gbk AUTO_INCREMENT=1 ;

DROP TABLE IF EXISTS t_demo_test2;
CREATE TABLE IF NOT EXISTS t_demo_test2(
    tid int(20) NOT NULL AUTO_INCREMENT,
    userid int(20),
    name varchar(50),
    class varchar(50),
    PRIMARY KEY(tid)
)ENGINE=InnoDB  DEFAULT CHARSET=gbk AUTO_INCREMENT=1 ;

insert into t_demo_test2(userid, name, class) values(11, 'fred', 'important class');

insert into t_demo_test1(userid, day, title, content) 
values(10,'2015-04-05 22:10:30','title1','content sinny write test')
,(11,'2015-04-05 22:10:30','title1','一种内容1')
,(12,'2015-04-06 22:10:30','title1','一种内容2')
,(13,'2015-04-07 22:10:30','title1','一种内容3')
,(14,'2015-04-08 22:10:30','title1','一种内容4');

2、直接构造临时表

CREATE TEMPORARY TABLE tmp_table SELECT userid, name FROM t_demo_test2;

3、在存储过程中使用临时表

DELIMITER //
DROP PROCEDURE IF EXISTS p_modify_content;
CREATE PROCEDURE p_modify_content
(
    IN v_mcontent varchar(10),
    OUT v_output varchar(100),
    OUT v_count int
)
BEGIN
    DECLARE tmpid int(20);
    DECLARE tname varchar(50);
    DECLARE tuserid int(20);
    DECLARE done INT DEFAULT FALSE;
    DECLARE cur CURSOR FOR SELECT t_demo_test1.userid, t_demo_test1.tid FROM t_demo_test1;    
    DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;
    OPEN cur;
    
    DROP TEMPORARY TABLE IF EXISTS usertmp;
 
    CREATE TEMPORARY TABLE usertmp(
        userid int(20),
        name varchar(50)
    ) ENGINE = MEMORY;
    
    myloop: LOOP
        FETCH cur INTO tuserid, tmpid;

        IF done THEN
            LEAVE myloop;
        END IF;
        
        SELECT t_demo_test2.name from t_demo_test2 WHERE userid = tuserid INTO tname;
        
        select tname;
        IF !isNull(tname) THEN    
            INSERT INTO usertmp VALUES(tuserid, tname);
        END IF;
        
        UPDATE t_demo_test1 SET content = concat(content,v_mcontent, tname) WHERE tid = tmpid;    
        SET done = FALSE;/* SELECT t_demo_test2.name from t_demo_test2 WHERE userid = tuserid INTO tname; 也会触发NOT FOUND */
        SET tname = null;
    END LOOP;
    SELECT COUNT(*) FROM usertmp INTO v_count;
    SET v_output = 'finished';
    CLOSE cur;
    DROP TEMPORARY TABLE IF EXISTS usertmp;
END
//

call p_modify_content("hello", @content, @count);
select @content;//输出finished
select @count;//输出1

4、临时表与内存表的区别

临时表，schema和数据都在内存中，创建表后，无法通过show tables看到该表，重新进入后无表、无数据；

内存表，schema在硬盘中，数据在内存中，创建表后可以在show tables看到该表，重新进入后有表，无数据； 

如下，建立内存表：

create table XXtable(
    。。。
)engine = HEAP;