Mysql 获取同一表中两列之间的引用数

我有一个问题，就是如何获取MySQL中两列之间某个值的出现次数

id1 col1 col2
2    5      3
3    3      4
4    2      1
5    1      3
6    null   2

我怎样才能得到下面两列之间发生的次数

value     occurrence
3                3
1                2
2                2
4                1
5                1

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您可以

联合所有列

，然后应用

计数

聚合函数：

SELECT   val, COUNT(*) AS occurrence
FROM     (SELECT col1 AS val
          FROM   mytable
          UNION ALL
          SELECT col2 AS val
          FROM   mytable) x
GROUP BY val
ORDER BY occurrence DESC

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根据实际数据（每个值的行数），预聚合可能更有效，只需尝试一下：

SELECT   val, SUM(occurrence) AS occurrence
FROM     (SELECT col1 AS val, COUNT(*) AS occurrence
          FROM   mytable
          UNION ALL
          SELECT col2 AS val, COUNT(*) AS occurrence
          FROM   mytable) x
GROUP BY val
ORDER BY occurrence DESC

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非常感谢你！我一直在努力，直到今天。