PostgreSQL：使数据显示在不同的行而不是同一行中

Question

我有这个查询：

SELECT
    SUM(CASE WHEN color = 'blue' THEN 1 ELSE 0 END) AS "Blue",
    SUM(CASE WHEN color = 'purple' THEN 1 ELSE 0 END) AS "Purple",
    SUM(CASE WHEN color = 'yellow' THEN 1 ELSE 0 END) AS "Yellow",
    SUM(CASE WHEN color= 'blue' THEN 1 WHEN color= 'yellow' THEN -1 END) AS "Mixed"
FROM table;

结果显示出一个有4列1行的表：

例如蓝色|紫色|黄色|混合

但是我希望结果显示在4行1列中，如下所示：

蓝色

• 蓝色
• 紫色
• 黄色
• 混合

有人知道在这种情况下我能做什么吗？我一直在为此工作。

Answer 1

这有点令人痛苦，因为混合值-简单的聚合是不够的。因此，请使用侧向联接取消枢纽：

SELECT v.*
FROM (SELECT SUM(CASE WHEN color = 'blue' THEN 1 ELSE 0 END) AS Blue,
             SUM(CASE WHEN color = 'purple' THEN 1 ELSE 0 END) AS Purple,
             SUM(CASE WHEN color = 'yellow' THEN 1 ELSE 0 END) AS Yellow,
             SUM(CASE WHEN color = 'blue' THEN 1 WHEN color = 'yellow' THEN -1 END) AS Mixed
      FROM t
     ) t CROSS JOIN LATERAL
     (VALUES ('Blue', blue), ('Purple', purple), ('Yellow', yellow), ('Mixed', mixed)
     ) v(which, val);

实际上，在Postgres中，您也可以这样写：

SELECT v.*
FROM (SELECT COUNT(*) FILTER (WHERE color = 'blue') AS Blue,
             COUNT(*) FILTER (WHERE color = 'purple') AS Purple,
             COUNT(*) FILTER (WHERE color = 'yellow') AS Yellow
      FROM t
     ) t CROSS JOIN LATERAL
     (VALUES ('Blue', blue), ('Purple', purple), ('Yellow', yellow), ('Mixed', blue - yellow)
     ) v(which, val)

Answer 2

对于前三个计数器，您可以group by color进行汇总并将UNION ALL用于Mixed：

SELECT color, COUNT(*) counter
WHERE color IN ('blue', 'purple', 'yellow') -- remove this line if these are all the possible colors
FROM table
GROUP BY color
UNION ALL
SELECT 'Mixed', SUM(CASE WHEN color = 'blue' THEN 1 WHEN color = 'yellow' THEN -1 END)
WHERE color IN ('blue', 'yellow')
FROM table

此查询将返回2列。如果您不希望第一栏，那么：

SELECT COUNT(*) counter
WHERE color IN ('blue', 'purple', 'yellow') -- remove this line if these are all the possible colors
FROM table
GROUP BY color
UNION ALL
SELECT SUM(CASE WHEN color = 'blue' THEN 1 WHEN color = 'yellow' THEN -1 END)
FROM table
WHERE color IN ('blue', 'yellow')