SQL 将行值转换为列标题

我有下表:

tableA
+-----------+--------+
| tableA_id |  code  |
+-----------+--------+
|         1 | code A |
|         2 | code B |
|         3 | code A |
|         3 | code C |
|         3 | code B |
|         4 | code A |
|         4 | code C |
|         4 | code B |
|         5 | code A |
|         5 | code C |
|         5 | code B |
+-----------+--------+

我想使用查询来显示代码 A、代码 B、代码 C 作为列标题，然后值将显示 tableA_id 条目是否在代码字段中包含该代码。所以像这样:

+-----------+------------------------------+
| tableA_id |  code A |  code B  |  code C |
+-----------+------------------------------+
|         1 |   yes   |          |         |
|         2 |         |   yes    |   yes   |
|         3 |   yes   |   yes    |   yes   |

etc...

你能在 SQL 中做到这一点吗？

最佳答案

使用条件聚合(可在不同方言之间移植):

SELECT tableA_id,
       MAX(CASE WHEN code ='code A' THEN 'yes' END) AS "code A",
       MAX(CASE WHEN code ='code B' THEN 'yes' END) AS "code B",
       MAX(CASE WHEN code ='code C' THEN 'yes' END) AS "code C"
FROM tableA
GROUP BY tableA_id;

SqlFiddleDemo_MySQL
SqlFiddleDemo_Postgresql

输出:

╔════════════╦═════════╦═════════╦════════╗
║ tableA_id  ║ code A  ║ code B  ║ code C ║
╠════════════╬═════════╬═════════╬════════╣
║         1  ║ yes     ║ (null)  ║ (null) ║
║         2  ║ (null)  ║ yes     ║ (null) ║
║         3  ║ yes     ║ yes     ║ yes    ║
║         4  ║ yes     ║ yes     ║ yes    ║
║         5  ║ yes     ║ yes     ║ yes    ║
╚════════════╩═════════╩═════════╩════════╝

有很多可能性(搜索):

PIVOT            -> SQL Server/Oracle
CROSSTAB         -> Postgresql
SELF OUTER JOIN  -> All
CONDITIONAL AGG  -> All
...

关于SQL 将行值转换为列标题，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/34655535/