mysql - 三个表的 JOIN 中的计数为零

Question

我有三张 table ；县、公墓和坟墓。 Graves 表中的 Marked 列是 bool 值。查询结果应包括每个墓地中“标记”坟墓的数量(包括零计数)。

    County
-------------------------
|  CountyID  | Name      |
-------------------------|
|   351      | Lee       |
|   352      | Glascock  |
-------------------------


   Cemetery
--------------------------------------
| CEMID  |  CountyID  | Name          |
--------------------------------------|
|  1     |      352   | Smith Family  |
|  2     |      352   | Adams Family  |
|  3     |      352   | Jones Family  |
|  4     |      351   | Harris Family |
--------------------------------------

   Graves
--------------------------------------
|  ID | CEMID |     Name     | Marked |
--------------------------------------|
|  1  |   1   | John Smith   |   Y    |
|  2  |   1   | Mary Smith   |   N    |
|  3  |   1   | Frank Smith  |   Y    |
|  4  |   1   | Jane Smith   |   N    |
|  5  |   2   | John Adams   |   Y    |
|  6  |   2   | Ed Adams     |   N    |
|  7  |   3   | Susan Jones  |   N    |
--------------------------------------

   Query Result
----------------------------------
|  CEMID |     Name       | Count |
----------------------------------|
|  1     |  Smith Family  |  2    |
|  2     |  Adams Family  |  1    |
|  2     |  Jones Family  |  0    |
----------------------------------|

My current SELECT statement:

SELECT Cemetery.Name, Cemetery.CEMID, COUNT(Grave.Marked) AS Count
         FROM Cemetery
         JOIN County
         ON Cemetery.CountyID = County.CountyID
         JOIN Grave
         ON Cemetery.CEMID = Grave.CEMID
         WHERE CountyID = 352
         AND Grave.Marked = TRUE
         GROUP BY Cemetery.CEMID;

此语句返回结果，但不包括琼斯家族墓地，因为标记计数为零。我知道“Grave.Marked = TRUE”位于错误的位置，但我应该在哪里限定计数以包含标记计数为零的墓地。

Answer 1

您想要使用左连接:

SELECT c.Name, c.CEMID, COUNT(g.Marked) AS Count
FROM Cemetery c LEFT JOIN
     Grave g
     ON c.CEMID = g.CEMID AND g.Marked = TRUE
WHERE c.CountyID = 352
GROUP BY c.CEMID;

对查询的更改:

• 不需要county 表。您可以从cemetary获取id。
• 连接现在是左连接。
• Marked 的条件已移至 on 子句，而不是 where 子句。
• 使用表别名可以使查询更易于编写和读取。