获取相同两列之间的出现次数 table

Question

我在如何获取 MySQL 中两列之间某个值的出现次数方面遇到问题。

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

我怎样才能像下面这样得到两列之间出现的次数？

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

Answer 1

您可以 union all 列，然后应用 count 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

Answer 2

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

获取相同两列之间的出现次数 table

get number of occurrences between two columns same table

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count