[MySQL]行列转换变化各种方法实现总结（行变列报表统计、列变行数据记录统计等）_MySQL

程序员文章站 2022-06-07 09:29:56

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前言：

mysql行列变化，最难的就是将多个列变成多行，使用的比较多的是统计学中行变列，列变行，没有找到现成的函数或者语句，所以自己写了存储过程，使用动态sql来实现，应用业务场景，用户每个月都有使用记录数录入一张表，一个月一个字段，所以表的字段是动态增长的，现在需要实时统计当前用户使用的总数量，如果你知道有多少个字段，那么可以用select c1+c2+c3+…. From tbname where tid=’111’;来实现，但是关键是这个都是动态的，所以在应用程序端来实现确实不适宜，可以放在数据库后台在存储过程里实现。

而且在行变成列中，如果要写单个sql来实现，列的数目就需要写死，因为如果不知道要展示成多少列的话，就需要用动态变量，而一条sql里面无法使用动态变量。但是可以使用sql块来实现动态的效果。

一，列变成行例子演示

1，准备测试数据

这是基础数据表，里面有多个字段wm201403……，现在需要把N个这样的列变成行数据。

USE csdn;
DROP TABLE IF EXISTS flow_table;
CREATE TABLE `flow_table` (
  `ID` INT(11) NOT NULL AUTO_INCREMENT,
  `Number` BIGINT(11) NOT NULL,
  `City` VARCHAR(10) NOT NULL,
  `wm201403` DECIMAL(7,2) DEFAULT NULL,
  `wm201404` DECIMAL(7,2) DEFAULT NULL,
  `wm201405` DECIMAL(7,2) DEFAULT NULL,
  `wm201406` DECIMAL(7,2) DEFAULT NULL,
  `wm201407` DECIMAL(7,2) DEFAULT NULL,
  `wm201408` DECIMAL(7,2) DEFAULT NULL,
  PRIMARY KEY (`ID`,`Number`)
) ENGINE=INNODB   DEFAULT CHARSET=utf8;

录入一批测试数据：

INSERT INTO flow_table(Number,City,wm201403,wm201404,wm201405,wm201406,wm201407,wm201408)SELECT 1,'shanghai',100.2,180.4,141,164,124,127;
INSERT INTO flow_table(Number,City,wm201403,wm201404,wm201405,wm201406,wm201407,wm201408)SELECT 2,'shanghai',110.23,180.34,141.23,104.78,124.67,127.45;
INSERT INTO flow_table(Number,City,wm201403,wm201404,wm201405,wm201406,wm201407,wm201408)SELECT 3,'beijing',123.23,110.34,131.33,154.58,154.67,167.45;
INSERT INTO flow_table(Number,City,wm201403,wm201404,wm201405,wm201406,wm201407,wm201408)SELECT 4,'hangzhou',0,110.34,131.33,154.58,154.67,0;
INSERT INTO flow_table(Number,City,wm201405,wm201406,wm201407,wm201408)SELECT 5,'hangzhou',131.33,154.58,154.67,0;

需要达到的统计效果是：

+--------+-----------+

| Number | total_num |

+--------+-----------+

| 1 | 836.60 |

| 2 | 788.70 |

| 3 | 841.60 |

| 4 | 550.92 |

| 5 | 440.58 |

+--------+-----------+

5 rows in set (0.00 sec)

2，存储过程遍历：

这个存储过程建立了2张临时表，查询测试表数据形成游标，遍历游标根据主键Number来调用pro_flow_modify存储过程进行行列变化。代码如下：

DELIMITER $$
DROP PROCEDURE IF EXISTS csdn.`proc_all_changes`$$
CREATE PROCEDURE csdn.proc_all_changes()
BEGIN
    DECLARE v_number BIGINT;
    DECLARE v_city VARCHAR(10);
    DECLARE _done INT DEFAULT 0; 
    
    /*定义游标*/
    DECLARE cur_all CURSOR FOR SELECT Number,City FROM csdn.`flow_table`;
    /**这里如果需要定义下当NOT FOUND的时候，EXIT退出游标遍历，不然如果设置成CONTINUE会一直执行下去。*/
    DECLARE EXIT HANDLER FOR NOT FOUND BEGIN SET _done=1;END;      
    
       /*建立临时表，存放所有字段的临时表*/
	DROP TABLE IF EXISTS flow_n_columns;
	CREATE TABLE `flow_n_columns` (
	  `column_name` VARCHAR(10) NOT NULL
	) ENGINE=INNODB DEFAULT CHARSET=utf8;

	/*存放最终变成行的数据表*/
	DROP TABLE IF EXISTS flow_tmp;
	CREATE TABLE `flow_tmp` (
	  `Number` INT(11) DEFAULT NULL,
	  `City` VARCHAR(10) DEFAULT NULL,
	  `wm_str` VARCHAR(10) DEFAULT NULL,
	  `Wm` DECIMAL(7,2) DEFAULT NULL
	) ENGINE=INNODB DEFAULT CHARSET=utf8;

    OPEN cur_all;
    REPEAT
        FETCH cur_all INTO v_number, v_city;
        IF NOT _done THEN 
		CALL csdn.pro_flow_modify(v_number,v_city);
        END IF; 
        UNTIL _done=1 END REPEAT;
    CLOSE cur_all; 
    	/*展示下所有的行转列的数据**/
	SELECT * FROM csdn.flow_tmp;      
END$$   
DELIMITER ;

3，行里变化存储过程

通过查询系统表information_schema.`COLUMNS`来获取测试表flow_table的所有列，然后写动态SQL，来把列的值录入到临时表flow_tmp中。

DELIMITER $$
DROP PROCEDURE IF EXISTS csdn.`pro_flow_modify`$$
CREATE PROCEDURE csdn.`pro_flow_modify`(p_Number INT,p_city VARCHAR(10))
BEGIN
	DECLARE v_column_name VARCHAR(10) DEFAULT '';
	DECLARE v_exe_sql VARCHAR(1000) DEFAULT '';
	DECLARE v_start_wm VARCHAR(10) DEFAULT '';
	DECLARE v_end_wm VARCHAR(10) DEFAULT '';
	DECLARE v_num  DECIMAL(10,2) DEFAULT 0;
	
	DECLARE i INT DEFAULT 1;
	DECLARE v_Number INT DEFAULT 0;
	SET v_Number=p_Number;
	
	DELETE FROM csdn.flow_n_columns;
	DELETE FROM csdn.flow_tmp WHERE Number=v_Number;
	
	
	/*把测试表flow_table的所有字段都录入字段临时表中，这样就达到了从列变成行的目的*/
	INSERT INTO flow_n_columns
	SELECT t.`COLUMN_NAME` FROM information_schema.`COLUMNS` t WHERE t.`TABLE_NAME`='flow_table' AND t.`TABLE_SCHEMA`='csdn' AND t.`COLUMN_NAME` NOT IN('ID','Number','City');
	SELECT column_name INTO v_column_name FROM csdn.flow_n_columns LIMIT 1;
	
	/*开始循环遍历字段临时表的字段数据，并且把字段值放入临时表flow_tmp里面*/
	WHILE i>0 DO
		SET v_exe_sql=CONCAT('INSERT INTO csdn.flow_tmp(Number,City,wm_str,Wm) select ',v_Number,',\'',p_city, '\',\'',v_column_name,'\',',v_column_name,' from csdn.flow_table WHERE flow_table.Number=',v_Number,';');
		SET @sql=v_exe_sql;
		PREPARE s1 FROM @sql;
		EXECUTE s1;
		DEALLOCATE PREPARE s1; 
		DELETE FROM csdn.flow_n_columns WHERE column_name=v_column_name;
		SELECT column_name INTO v_column_name FROM csdn.flow_n_columns LIMIT 1;
		SELECT COUNT(1) INTO i FROM csdn.flow_n_columns ;
		DELETE FROM csdn.flow_tmp WHERE Wm=0;
	END WHILE;

	/*由于触发器是不支持动态sql，所以不能使用while循环，动态遍历所有统计列的，只能写死列了，如下所示：
	现在一个个insert只能写死了, flow_table表有多少个统计列就写多少个insert sql，以后新添加一个列，就在这里新添加一条insertsql语句
	INSERT INTO flow_tmp(Number,City,wm_str,Wm) SELECT v_Number,p_city,'wm201403',wm201403 FROM flow_table WHERE Number=v_Number ;
	INSERT INTO flow_tmp(Number,City,wm_str,Wm) SELECT v_Number,p_city,'wm201404',wm201404 FROM flow_table WHERE Number=v_Number ;
	INSERT INTO flow_tmp(Number,City,wm_str,Wm) SELECT v_Number,p_city,'wm201405',wm201405 FROM flow_table WHERE Number=v_Number ;
	INSERT INTO flow_tmp(Number,City,wm_str,Wm) SELECT v_Number,p_city,'wm201406',wm201406 FROM flow_table WHERE Number=v_Number ;
	INSERT INTO flow_tmp(Number,City,wm_str,Wm) SELECT v_Number,p_city,'wm201407',wm201407 FROM flow_table WHERE Number=v_Number ;
	INSERT INTO flow_tmp(Number,City,wm_str,Wm) SELECT v_Number,p_city,'wm201408',wm201408 FROM flow_table WHERE Number=v_Number ;
	*/
	
	/*清除掉不数据=0的列*/
	DELETE FROM csdn.flow_tmp WHERE Wm=0 OR Wm IS NULL;
	
	SELECT wm_str INTO v_start_wm FROM csdn.flow_tmp WHERE Number=v_Number ORDER BY wm_str ASC LIMIT 1;
	SELECT wm_str INTO v_end_wm FROM csdn.flow_tmp WHERE Number=v_Number ORDER BY wm_str DESC LIMIT 1;
	SELECT SUM(Wm) INTO v_num FROM csdn.flow_tmp WHERE Number=v_Number;		

    END$$

DELIMITER ;

4，列变行结果展示

临时表的所有数据：

mysql> SELECT * FROM csdn.flow_tmp;
+--------+----------+----------+--------+
| Number | City     | wm_str   | Wm     |
+--------+----------+----------+--------+
|      1 | shanghai | wm201403 | 100.20 |
|      1 | shanghai | wm201404 | 180.40 |
|      1 | shanghai | wm201405 | 141.00 |
|      1 | shanghai | wm201406 | 164.00 |
|      1 | shanghai | wm201407 | 124.00 |
|      1 | shanghai | wm201408 | 127.00 |
|      2 | shanghai | wm201403 | 110.23 |
|      2 | shanghai | wm201404 | 180.34 |
|      2 | shanghai | wm201405 | 141.23 |
|      2 | shanghai | wm201406 | 104.78 |
|      2 | shanghai | wm201407 | 124.67 |
|      2 | shanghai | wm201408 | 127.45 |
|      3 | beijing  | wm201403 | 123.23 |
|      3 | beijing  | wm201404 | 110.34 |
|      3 | beijing  | wm201405 | 131.33 |
|      3 | beijing  | wm201406 | 154.58 |
|      3 | beijing  | wm201407 | 154.67 |
|      3 | beijing  | wm201408 | 167.45 |
|      4 | hangzhou | wm201404 | 110.34 |
|      4 | hangzhou | wm201405 | 131.33 |
|      4 | hangzhou | wm201406 | 154.58 |
|      4 | hangzhou | wm201407 | 154.67 |
|      5 | hangzhou | wm201405 | 131.33 |
|      5 | hangzhou | wm201406 | 154.58 |
|      5 | hangzhou | wm201407 | 154.67 |
+--------+----------+----------+--------+
25 rows in set (0.00 sec)
mysql>

统计每个用户的使用总量为：

mysql> SELECT Number,SUM(Wm) 'total_num' FROM flow_tmp GROUP BY Number ORDER BY Number;
+--------+-----------+
| Number | total_num |
+--------+-----------+
|      1 |    836.60 |
|      2 |    788.70 |
|      3 |    841.60 |
|      4 |    550.92 |
|      5 |    440.58 |
+--------+-----------+
5 rows in set (0.00 sec)

mysql>

二，行变列例子演示

1，准备测试数据

USE csdn;
DROP TABLE IF EXISTS csdn.tb;
CREATE TABLE tb(`cname` VARCHAR(10),cource VARCHAR(10),score INT) ENGINE=INNODB;

INSERT INTO tb VALUES('张三','语文',74);
INSERT INTO tb VALUES('张三','数学',83);
INSERT INTO tb VALUES('张三','物理',93);
INSERT INTO tb VALUES('李四','语文',74);
INSERT INTO tb VALUES('李四','数学',84);
INSERT INTO tb VALUES('李四','物理',94);

SELECT * FROM tb;

需要得到的结果是：

+--------------------+--------+--------+--------+-----------+--------------+

| 姓名 | 语文 | 数学 | 物理 | 总成绩 | 平均成绩 |

+--------------------+--------+--------+--------+-----------+--------------+

| 张三 | 74.00 | 83.00 | 93.00 | 250.00 | 83.33 |

| 李四 | 74.00 | 84.00 | 94.00 | 252.00 | 84.00 |

| 总成绩平均数 | 74.00 | 83.50 | 93.50 | 251.00 | 83.67 |

+--------------------+--------+--------+--------+-----------+--------------+

2，利用SUM(IF()) 生成列 + WITH ROLLUP 生成汇总行,并利用 IFNULL将汇总行标题显示为 Total_num

SQL代码块如下：

SELECT cname AS "姓名",
	SUM(IF(cource="语文",score,0)) AS "语文",
	SUM(IF(cource="数学",score,0)) AS "数学",
	SUM(IF(cource="物理",score,0)) AS "物理",
	SUM(score) AS "总成绩",
	ROUND(AVG(score),2) AS "平均成绩"
FROM tb 
GROUP BY cname
UNION ALL
SELECT
	"总成绩平均数",
	ROUND(AVG(`语文`),2) , ROUND(AVG(`数学`),2), ROUND(AVG(`物理`),2), ROUND(AVG(`总成绩`),2), ROUND(AVG(`平均成绩`),2)
FROM(
	SELECT "all",cname AS "姓名",
		SUM(IF(cource="语文",score,0)) AS "语文",
		SUM(IF(cource="数学",score,0)) AS "数学",
		SUM(IF(cource="物理",score,0)) AS "物理",
		SUM(score) AS "总成绩",
		AVG(score) AS "平均成绩"
	FROM tb 
	GROUP BY cname
)tb2 
GROUP BY tb2.all;

执行结果正确，如下所示：

+--------------------+--------+--------+--------+-----------+--------------+

| 姓名 | 语文 | 数学 | 物理 | 总成绩 | 平均成绩 |

+--------------------+--------+--------+--------+-----------+--------------+

| 张三 | 74.00 | 83.00 | 93.00 | 250.00 | 83.33 |

| 李四 | 74.00 | 84.00 | 94.00 | 252.00 | 84.00 |

| 总成绩平均数 | 74.00 | 83.50 | 93.50 | 251.00 | 83.67 |

+--------------------+--------+--------+--------+-----------+--------------+

3，利用max(CASE ... WHEN ... THEN .. ELSE END) AS "语文"的方式来实现

SQL代码如下：

SELECT 
	cname AS "姓名",
	MAX(CASE cource WHEN "语文" THEN score ELSE 0 END) AS "语文", 
	MAX(CASE cource WHEN "数学" THEN score ELSE 0 END) AS "数学", 
	MAX(CASE cource WHEN "物理" THEN score ELSE 0 END) AS "物理", 
	SUM(score) AS "总成绩",
	ROUND(AVG(score) ,2) AS "平均成绩"
FROM tb 
GROUP BY `cname`
UNION ALL
SELECT
	"总成绩平均数",
	ROUND(AVG(`语文`),2) , ROUND(AVG(`数学`),2), ROUND(AVG(`物理`),2), ROUND(AVG(`总成绩`),2), ROUND(AVG(`平均成绩`),2)
FROM(	SELECT 'all' ,  
		cname AS "姓名",
		MAX(CASE cource WHEN "语文" THEN score ELSE 0 END) AS "语文", 
		MAX(CASE cource WHEN "数学" THEN score ELSE 0 END) AS "数学", 
		MAX(CASE cource WHEN "物理" THEN score ELSE 0 END) AS "物理", 
		SUM(score) AS "总成绩",
		ROUND(AVG(score) ,2) AS "平均成绩"
	FROM tb 
	GROUP BY `cname` 
)tb2 GROUP BY tb2.all

执行结果正确，如下所示：

+--------------------+--------+--------+--------+-----------+--------------+

| 姓名 | 语文 | 数学 | 物理 | 总成绩 | 平均成绩 |

+--------------------+--------+--------+--------+-----------+--------------+

| 张三 | 74.00 | 83.00 | 93.00 | 250.00 | 83.33 |

| 李四 | 74.00 | 84.00 | 94.00 | 252.00 | 84.00 |

| 总成绩平均数 | 74.00 | 83.50 | 93.50 | 251.00 | 83.67 |

+--------------------+--------+--------+--------+-----------+--------------+

4，利用 WITH rollup结果不符合

SQL代码如下：

  SELECT IFNULL(cname,'总平均数') AS "姓名",
	MAX(CASE cource WHEN "语文" THEN score ELSE 0 END) AS "语文", 
	MAX(CASE cource WHEN "数学" THEN score ELSE 0 END) AS "数学", 
	MAX(CASE cource WHEN "物理" THEN score ELSE 0 END) AS "物理", 
	ROUND(AVG(score),2) AS "总成绩",
	ROUND(AVG(avg_score),2) AS "平均成绩" 
    FROM(
	SELECT   
		cname ,
		IFNULL(cource,'total') cource,
		SUM(score) AS score,
		ROUND(AVG(score) ,2) AS avg_score
	FROM tb 
	GROUP BY `cname`,cource WITH ROLLUP HAVING cname IS NOT NULL
	)tb2 
	GROUP BY  tb2.cname WITH ROLLUP;

mysql>     SELECT IFNULL(cname,'总平均数') AS "姓名",
    -> MAX(CASE cource WHEN "语文" THEN score ELSE 0 END) AS "语文", 
    -> MAX(CASE cource WHEN "数学" THEN score ELSE 0 END) AS "数学", 
    -> MAX(CASE cource WHEN "物理" THEN score ELSE 0 END) AS "物理", 
    -> ROUND(AVG(score),2) AS "总成绩",
    -> ROUND(AVG(avg_score),2) AS "平均成绩" 
    ->     FROM(
    -> SELECT   
    -> 
Display ALL 793 possibilities? (Y OR n) 
    -> cname ,
    -> 
Display ALL 793 possibilities? (Y OR n) 
    -> IFNULL(cource,'total') cource,
    -> 
Display ALL 793 possibilities? (Y OR n) 
    -> SUM(score) AS score,
    -> 
Display ALL 793 possibilities? (Y OR n) 
    -> ROUND(AVG(score) ,2) AS avg_score
    -> FROM tb 
    -> GROUP BY `cname`,cource WITH ROLLUP HAVING cname IS NOT NULL
    -> )tb2 
    -> GROUP BY  tb2.cname WITH ROLLUP;
+--------------+--------+--------+--------+-----------+--------------+
| 姓名         | 语文   | 数学   | 物理   | 总成绩    | 平均成绩     |
+--------------+--------+--------+--------+-----------+--------------+
| 张三         |     74 |     83 |     93 |    125.00 |        83.33 |
| 李四         |     74 |     84 |     94 |    126.00 |        84.00 |
| 总平均数     |     74 |     84 |     94 |    125.50 |        83.67 |
+--------------+--------+--------+--------+-----------+--------------+
3 ROWS IN SET, 1 warning (0.00 sec)
mysql>

总结： WITH rollup中对求列的总数是OK的，但是求列的平均数有偏差，这里场景使用不是恰当。

5，使用动态SQL来实现

SQL代码块如下：

/*仅仅班级成员部分*/
SET @a=''; 
SELECT @a:=CONCAT(@a,'SUM(IF(cource=\'',cource,'\'',',score,0)) AS ',cource,',') FROM (SELECT DISTINCT cource FROM tb) A;
SET @a=CONCAT(@a,"ROUND(AVG(score) ,2) AS \"平均成绩\"");
SET @b=CONCAT('SELECT IFNULL(cname,\'总成绩\'),',LEFT(@a,LENGTH(@a)-1),' ,SUM(score) AS \"总成绩\" FROM tb GROUP BY cname ');

/*班级成员总计部分**/
SET @a2="";
SET @b2=CONCAT('SELECT "all",IFNULL(cname,\'总成绩\'),',LEFT(@a,LENGTH(@a)-1),' ,SUM(score) AS \"总成绩\" FROM tb GROUP BY cname ');
SELECT @a2:=CONCAT(@a2,'ROUND(AVG(`',cource,'`),2),') FROM (SELECT DISTINCT cource FROM tb) A;
SET @a2=CONCAT(@a2," ROUND(AVG(`平均成绩`),2),ROUND(AVG(`总成绩`),2) ");
SET @c=CONCAT("SELECT \"班级平均数\",",LEFT(@a2,LENGTH(@a)-1)," FROM(",@b2,")tb2 GROUP BY tb2.all;");
SET @d=CONCAT(@b," UNION ALL ",@c);

PREPARE stmt1 FROM @d;
EXECUTE stmt1;

查看执行结果如下，已经达到效果：

mysql> /*仅仅班级成员部分*/
mysql> SET @a=''; 
QUERY OK, 0 ROWS affected (0.00 sec)

mysql> SELECT @a:=CONCAT(@a,'SUM(IF(cource=\'',cource,'\'',',score,0)) AS ',cource,',') FROM (SELECT DISTINCT cource FROM tb) A;
+-----------------------------------------------------------------------------------------------------------------------------------+
| @a:=CONCAT(@a,'SUM(IF(cource=\'',cource,'\'',',score,0)) AS ',cource,',')                                                         |
+-----------------------------------------------------------------------------------------------------------------------------------+
| SUM(IF(cource='语文',score,0)) AS 语文,                                                                                           |
| SUM(IF(cource='语文',score,0)) AS 语文,SUM(IF(cource='数学',score,0)) AS 数学,                                                    |
| SUM(IF(cource='语文',score,0)) AS 语文,SUM(IF(cource='数学',score,0)) AS 数学,SUM(IF(cource='物理',score,0)) AS 物理,             |
+-----------------------------------------------------------------------------------------------------------------------------------+
3 ROWS IN SET (0.00 sec)

mysql> SET @a=CONCAT(@a,"ROUND(AVG(score) ,2) AS \"平均成绩\"");
QUERY OK, 0 ROWS affected (0.00 sec)

mysql> SET @b=CONCAT('SELECT IFNULL(cname,\'总成绩\'),',LEFT(@a,LENGTH(@a)-1),' ,SUM(score) AS \"总成绩\" FROM tb GROUP BY cname ');
QUERY OK, 0 ROWS affected (0.00 sec)

mysql> 
mysql> /*班级成员总计部分**/
mysql> SET @a2="";
QUERY OK, 0 ROWS affected (0.00 sec)

mysql> SET @b2=CONCAT('SELECT "all",IFNULL(cname,\'总成绩\'),',LEFT(@a,LENGTH(@a)-1),' ,SUM(score) AS \"总成绩\" FROM tb GROUP BY cname ');
QUERY OK, 0 ROWS affected (0.00 sec)

mysql> SELECT @a2:=CONCAT(@a2,'ROUND(AVG(`',cource,'`),2),') FROM (SELECT DISTINCT cource FROM tb) A;
+-----------------------------------------------------------------------+
| @a2:=CONCAT(@a2,'ROUND(AVG(`',cource,'`),2),')                        |
+-----------------------------------------------------------------------+
| ROUND(AVG(`语文`),2),                                                 |
| ROUND(AVG(`语文`),2),ROUND(AVG(`数学`),2),                            |
| ROUND(AVG(`语文`),2),ROUND(AVG(`数学`),2),ROUND(AVG(`物理`),2),       |
+-----------------------------------------------------------------------+
3 ROWS IN SET (0.00 sec)

mysql> SET @a2=CONCAT(@a2," ROUND(AVG(`平均成绩`),2),ROUND(AVG(`总成绩`),2) ");
QUERY OK, 0 ROWS affected (0.00 sec)

mysql> SET @c=CONCAT("SELECT \"班级平均数\",",LEFT(@a2,LENGTH(@a)-1)," FROM(",@b2,")tb2 GROUP BY tb2.all;");
QUERY OK, 0 ROWS affected (0.00 sec)

mysql> SET @d=CONCAT(@b," UNION ALL ",@c);
QUERY OK, 0 ROWS affected (0.00 sec)

mysql> 
mysql> PREPARE stmt1 FROM @d;
QUERY OK, 0 ROWS affected (0.00 sec)
Statement prepared

mysql> EXECUTE stmt1;
+---------------------------+--------+--------+--------+--------------+-----------+
| IFNULL(cname,'总成绩')    | 语文   | 数学   | 物理   | 平均成绩     | 总成绩    |
+---------------------------+--------+--------+--------+--------------+-----------+
| 张三                      |  74.00 |  83.00 |  93.00 |        83.33 |    250.00 |
| 李四                      |  74.00 |  84.00 |  94.00 |        84.00 |    252.00 |
| 班级平均数                |  74.00 |  83.50 |  93.50 |        83.67 |    251.00 |
+---------------------------+--------+--------+--------+--------------+-----------+
3 ROWS IN SET (0.00 sec)
mysql>

参考文章地址：http://blog.chinaunix.net/uid-7692530-id-2567582.html