在 MYSQL 中使用 COUNT 后仅一行

Question

不知道为什么我只得到错误计数的一行！

以下是工作代码：

SELECT project_name, sub_project_name
FROM projects, sub_projects
WHERE projects.project_id = sub_projects.projects_project_id

结果：

project_name         sub_project_name
Bakken               Ghost fracture Study
Bakken               Bakken Mylo QAQC
Bossier              Doyle Boles K No.1
Eagleford            Kennedy Unit#1H
Eagleford            Wehmeyer Unit #1
Niobrara             Crow Valley 7-62-32-1M
Poland               Poland
Woodford             Ridenour Phase 2
Woodford             Teague 1-14H

每个 sub_project 都有多个表，我基本上是想查看其中一个表中的列是否有任何包含非 Null 值的行，然后在每个 sub_project 的另一列中的 sub_project 旁边显示该计数。

这是我查询以下语句时得到的结果：

SELECT projects.project_name, sub_projects.sub_project_name, COUNT( bl.bl_por ) AS porosity
FROM projects, sub_projects
LEFT JOIN bl ON sub_projects.sub_project_id = bl.sub_project_id
WHERE projects.project_id = sub_projects.projects_project_id

结果：

project_name         sub_project_name          porosity
Bakken               Ghost fracture Study      99

第一行中每个 sub_project 的所有行，仅此而已。

这里有什么问题？

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编辑：

Erwin 的解决方案成功了。但是我的表中没有sub_project_id。我只是将它添加到一个表中以使其更易于测试。

所以我使用 Erwin 的 GROUP BY 建议来编辑我的语句，我得到了正确的矩阵形状但不是正确的计数。计数还差得很远。

SELECT p.project_name, sp.sub_project_name, COUNT( bl.bl_por ) AS porosity
FROM projects p, sub_projects sp, wells w, cores c, samples s, inputs i
LEFT JOIN bl ON i.inputs_id = bl.inputs_inputs_id
WHERE p.project_id = sp.projects_project_id
AND s.sample_id = i.samples_sample_id
AND c.core_id = s.cores_core_id
AND sp.sub_project_id = c.sub_projects_has_wells_sub_projects_sub_project_id
GROUP BY p.project_name, sp.sub_project_name

结果：

project_name    sub_project_name        porosity
Bakken          Bakken Mylo QAQC        147
Bakken          Ghost fracture Study    252
Bossier         Doyle Boles K No.1      189
Eagleford       Kennedy Unit#1H         294
Eagleford       Wehmeyer Unit #1        0
Niobrara        Crow Valley 7-62-32-1M  0
Poland          Poland                  714
Woodford        Ridenour Phase 2        483
Woodford        Teague 1-14H            0

正确的结果应该是：

Bakken          Bakken Mylo QAQC        7
Bakken          Ghost fracture Study    12
Bossier         Doyle Boles K No.1      9
Eagleford       Kennedy Unit#1H         14
Eagleford       Wehmeyer Unit #1        0
Niobrara        Crow Valley 7-62-32-1M  0
Poland          Poland                  34
Woodford        Ridenour Phase 2        23
Woodford        Teague 1-14H            0

Answer 1

尝试显式的JOIN 条件和显式的GROUP BY（虽然mysql 允许在后面跳过）。

SELECT p.project_name, sp.sub_project_name, COUNT(bl.bl_por) AS porosity
FROM   projects p
JOIN   sub_projects sp ON p.project_id = sp.projects_project_id
LEFT   JOIN bl ON sp.sub_project_id = bl.sub_project_id
GROUP  BY p.project_name, sp.sub_project_name;

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回答附加问题：

再次尝试正确的 SQL 语法，使用显式 JOIN 和连接条件：

SELECT p.project_name, sp.sub_project_name, COUNT(bl.bl_por) AS porosity
FROM   projects p
JOIN   sub_projects sp ON p.project_id = sp.projects_project_id
JOIN   wells w -- no JOIN condition? Results in cross join.
JOIN   cores c ON sp.sub_project_id = c.sub_projects_has_wells_sub_projects_sub_project_id
JOIN   samples s ON c.core_id = s.cores_core_id
JOIN   inputs i ON s.sample_id = i.samples_sample_id
LEFT   JOIN bl ON i.inputs_id = bl.inputs_inputs_id
GROUP  BY p.project_name, sp.sub_project_name

通过这种方式，您会立即注意到表wells 是无条件加入的。这会导致交叉连接：左侧的每一行都与右侧的每一行一起扩展，从而产生很多行。可能是您计数过多的原因。

另外请注意，count 只计算非空值。不计算 bl.bl_por IS NULL 的任何行。如果你真的想计算行数，你可以使用count(bl.*)。