SELECT COUNT(DISTINCT [name]) from several tables

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爱一瞬间的悲伤
爱一瞬间的悲伤 2021-02-06 08:28

I can perform the following SQL Server selection of distinct (or non-repeating names) from a column in one table like so:

SELECT COUNT(DISTINCT [Name]) FROM [MyT         


        
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  • 2021-02-06 08:57

    In case you have different amounts of columns per table, like:

    • table1 has 3 columns,
    • table2 has 2 columns,
    • table3 has 1 column

    And you want to count the amount of distinct values of different column names, what it was useful to me in AthenaSQL was to use CROSS JOIN since your output would be only one row, it would be just 1 combination:

    SELECT * FROM (
    SELECT COUNT(DISTINCT name1) as amt_name1,
           COUNT(DISTINCT name2) as amt_name2,
           COUNT(DISTINCT name3) as amt_name3,
    FROM table1 ) t1
    CROSS JOIN
    (SELECT COUNT(DISTINCT name4) as amt_name4,
            COUNT(DISTINCT name5) as amt_name5,
            MAX(t3.amt_name6) as amt_name6
     FROM table2
     CROSS JOIN
     (SELECT COUNT(DISTINCT name6) as amt_name6
      FROM table3) t3) t2
    

    Would return a table with one row and their counts:

    amt_name1 | amt_name2 | amt_name3 | amt_name4 | amt_name5 | amt_name6
        4123  |    675    |    564    |    2346   |   18667   |    74567
    
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  • 2021-02-06 09:07

    After the clarification, use:

      SELECT x.name, COUNT(x.[name])
        FROM (SELECT [name]
                FROM [MyTable]
              UNION ALL
              SELECT [name]
                FROM [MyTable2]
              UNION ALL
              SELECT [name]
                FROM [MyTable3]) x
    GROUP BY x.name
    

    If I understand correctly, use:

      SELECT x.name, COUNT(DISTINCT x.[name])
        FROM (SELECT [name]
                FROM [MyTable]
              UNION ALL
              SELECT [name]
                FROM [MyTable2]
              UNION ALL
              SELECT [name]
                FROM [MyTable3]) x
    GROUP BY x.name
    

    UNION will remove duplicates; UNION ALL will not, and is faster for it.

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  • 2021-02-06 09:16

    EDIT: Had to change after seeing recent comment.

    Does this give you what you want? This gives a count for each person after combining the rows from all tables.

    SELECT [NAME], COUNT(*) as TheCount
    FROM
        (
         SELECT [Name] FROM [MyTable1]
         UNION ALL
         SELECT [Name] FROM [MyTable2]
         UNION ALL
         SELECT [Name] FROM [MyTable3]
         ) AS [TheNames]
    GROUP BY [NAME]
    
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  • 2021-02-06 09:20

    Here's another way:

    SELECT x.name, SUM(x.cnt)
    FROM ( SELECT [name], COUNT(*) AS cnt
           FROM [MyTable]
           GROUP BY [name]
         UNION ALL
           SELECT [name], COUNT(*) AS cnt
           FROM [MyTable2]
           GROUP BY [name]
         UNION ALL
           SELECT [name], COUNT(*) AS cnt
           FROM [MyTable3]
           GROUP BY [name]
         ) AS x
    GROUP BY x.name
    
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