现在需求是将表t1转化为表t2，要求仅查询一次t1表。 附1是表ddl语句，附2是使用union all写的SQL，由于多次查询表t1，所以当t1中指标很多（例如h_score、w_score、x_score....）的情况下不适用,寻求最优解。
表t1如图所示：
表t1
表t2
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附1
DROP TABLE IF EXISTS `t1`;
CREATE TABLE `t1` (
`user_name` varchar(30) CHARACTER SET utf8 COLLATE utf8_bin DEFAULT NULL,
`course` varchar(30) CHARACTER SET utf8 COLLATE utf8_bin DEFAULT NULL,
`h_score` int(10) DEFAULT NULL,
`w_score` int(10) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
BEGIN;
INSERT INTO `t1` VALUES ('张三', '英语', 50, 60);
INSERT INTO `t1` VALUES ('张三', '俄语', 70, 80);
INSERT INTO `t1` VALUES ('张三', '日语', 80, 90);
INSERT INTO `t1` VALUES ('李四', '英语', 45, 55);
INSERT INTO `t1` VALUES ('李四', '俄语', 65, 75);
INSERT INTO `t1` VALUES ('李四', '日语', 85, 95);
COMMIT; 附2
select user_name, 'h_score' as d, sum(case when course = '英语' then h_score else 0 end) as '英语', sum(case when course = '俄语' then h_score else 0 end) as '俄语', sum(case when course = '日语' then h_score else 0 end) as '日语' from t1 GROUP BY user_name union all select user_name, 'wscore' as d, sum(case when course = '英语' then w_score else 0 end) as '英语', sum(case when course = '俄语' then w_score else 0 end) as '俄语', sum(case when course = '日语' then w_score else 0 end) as '日语' from t1 GROUP BY user_name
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原文链接：https://blog.csdn.net/qq_35468937/article/details/105496867
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