使用 INNER JOIN 对 GROUP BY 中的单独 WHERE 子句求和

Question

我有两个 mysql 表，一个包含医院列表，另一个包含指标列表。我试图通过在医院表上执行 GROUP BY 和与度量表执行 INNER JOIN 来计算全州范围的汇总统计信息。我正在汇总表中寻找多个条件进行汇总。这就是我的数据库和查询现在的样子...

# create the hospital table and add 4 hospitals from 2 states
CREATE TABLE IF NOT EXISTS hospital (
    hospital_id INT NOT NULL AUTO_INCREMENT,
    name VARCHAR(255),
    state VARCHAR(2),
    PRIMARY KEY (hospital_id)
);
INSERT INTO hospital (name, state)
VALUES ('A', 'AL'), ('B', 'AL'), ('C', 'AK'), ('D', 'AK');


# create metrics table and add 2 metrics per hospital
CREATE TABLE IF NOT EXISTS metric (
    metric_id INT NOT NULL AUTO_INCREMENT,
    hospital VARCHAR(255),
    name VARCHAR(255),
    value INT,
    PRIMARY KEY (metric_id)
);
INSERT INTO metric (hospital, name, value)
VALUES ('A', 'total male patients', 100),
       ('A', 'total female patients', 110),
       ('B', 'total male patients', 55),
       ('B', 'total female patients', 50),
       ('C', 'total male patients', 10),
       ('C', 'total female patients', 7), 
       ('D', 'total male patients', 200),
       ('D', 'total female patients', 170);

# compute statewide metrics
SELECT
    hospital.state,
    metric.name,
    SUM(metric.value) as total
FROM
    hospital
INNER JOIN
    metric
ON
    hospital.name = metric.hospital
WHERE
    (metric.name = 'total male patients' OR metric.name = 'total female patients')
GROUP BY
    hospital.state,
    metric.name;

这是输出...

+-------+-----------------------+-------+
| state | name                  | total |
+-------+-----------------------+-------+
| AK    | total female patients |   177 |
| AK    | total male patients   |   210 |
| AL    | total female patients |   160 |
| AL    | total male patients   |   155 |
+-------+-----------------------+-------+

有没有办法重构我的查询，以便每个状态只得到一行，每个条件只得到一列。比如……

+-------+---------------------+-----------------------+
| state | total male patients | total female patients |
+-------+---------------------+-----------------------+
| AK    | 210                 |       177             |
| AL    | 155                 |       160             |
+-------+---------------------+-----------------------+

谢谢！

Answer 1

您可以使用条件聚合：

SELECT h.state,
       SUM(CASE WHEN m.name = 'total male patients'
                THEN m.value ELSE 0
           END) AS total_male,
       SUM(CASE WHEN m.name = 'total female patients'
                THEN m.value ELSE 0
           END) AS total_female
FROM hospital h INNER JOIN
     metric m
     ON h.name = m.hospital
GROUP BY h.state

请注意，我添加了表别名。我限定了有意义的列名。当您有多个表在查询时，您应该限定所有列名。

Answer 2

您可以在选择 2 列中的值时使用大小写

  SELECT
      state,
      Item,
      SUM(case when item = 'Total Veterans served' then Value else 0 end ) as total_Veterans_served, 
      SUM(case when item = 'N with PTSD'  then Value else 0 end ) as total_N_with_PTSD, 
  FROM va_location
  INNER JOIN patient_2015 ON va_location.facility_id = patient_2015.Location
  WHERE Category = 'Station-Level Stats'
  AND ValueType = 'Number'
  AND   (Item = 'Total Veterans served' OR Item = 'N with PTSD')
  GROUP BY
      state,
      Item