SQL 查询以显示列中的 table 列项目？

Question

我在数据库中有一个 table，如下所示：

-----------------------------
| ID | Item1 | Item2 | Item3|
-----------------------------
| 1  | A1    | B3    | C1   |
| 2  | A2    | B3    | C2   |
| 3  | A1    | B2    | C2   |
| 4  | A2    | B1    | C1   |
-----------------------------

我想显示 table 个项目，其计数如下：

---------------
| item | count|
---------------
| A1   | 2    | 
| A2   | 2    |
| B1   | 1    |
| B2   | 1    |
| B3   | 2    |
| C1   | 2    |
| C2   | 2    |
---------------

可能吗？如果是，查询如何？到目前为止，我只能像这样为每个项目一一显示它：

------------------  
| Item1  | Count |  
------------------
SELECT COUNT(*) AS `Count` , `Item1` FROM `table` GROUP BY `Item1`; 

------------------  
| Item2  | Count |  
------------------
SELECT COUNT(*) AS `Count` , `Item2` FROM `table` GROUP BY `Item2`;

------------------  
| Item3  | Count |  
------------------
SELECT COUNT(*) AS `Count` , `Item3` FROM `table` GROUP BY `Item3`;

Answer 1

使用union all和group by:

select item, count(*)
from ((select item1 as item from `table`) union all
      (select item2 as item from `table`) union all
      (select item3 as item from `table`)
     ) t
group by item
order by item;

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