在 MYSQL 中重新映射行答案

【问题标题】：Re-mapping rows in MYSQL在 MYSQL 中重新映射行
【发布时间】：2018-05-14 10:04:48
【问题描述】：

我有这样的表：

uid |确定 |价值 1 | 1 | nameOf1 1 | 2 |姓氏Of1 1 | 3 |国家/地区 2 | 1 | nameOf2 2 | 2 |姓Of2

并且需要像这样转换/选择它：

uid |姓名 |姓氏 |国家 1 | nameOf1 |姓氏Of1 |国家/地区 2 | nameOf2 |姓Of2 | ..

【问题讨论】：

mysql pivot query results with GROUP BY 的可能重复项

标签： mysql remap

【解决方案1】：

尝试类似：

SELECT t1.uid, t2.value AS name, t3.value AS surname, t4.value AS country FROM table t1
  LEFT JOIN table t2 on t2.uid = t1.uid AND t2.fid = 1
  LEFT JOIN table t3 on t3.uid = t1.uid AND t3.fid = 2
  LEFT JOIN table t4 on t4.uid = t1.uid AND t4.fid = 3
  GROUP BY t1.uid;

或：

SELECT DISTINCT t1.uid, t2.value AS name, t3.value AS surname, t4.value AS country FROM table t1
  LEFT JOIN table t2 on t2.uid = t1.uid AND t2.fid = 1
  LEFT JOIN table t3 on t3.uid = t1.uid AND t3.fid = 2
  LEFT JOIN table t4 on t4.uid = t1.uid AND t4.fid = 3;

【讨论】：

我对 280,000 条记录进行了测试（20% 的国家/地区为空）。没有索引，两个查询都太慢了。他们几乎不工作。然后我在两列上创建了一个主索引：uid 和 fid。并且这两个查询或多或少都运行得很快。

【解决方案2】：

这有点复杂，但你可以这样做：

select distinct uid as uid,
   (select value from dataTable as table1 
           where table1.uid=dataTable.uid and fid = 1) as name,
   (select value from dataTable as table2 
           where table2.uid=dataTable.uid and fid = 2) as surname,
   (select value from dataTable as table3 
           where table3.uid=dataTable.uid and fid = 3) as country
from dataTable group by uid

SQLFiddle

【讨论】：