SELECT COUNT(DISTINCT [name]) from several tables

问题

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 [MyTable]

But what if I have more than one table (all these tables contain the name field called [Name]) and I need to know the count of non-repeating names in two or more tables.

If I run something like this:

SELECT COUNT(DISTINCT [Name]) FROM [MyTable1], [MyTable2], [MyTable3]

I get an error, "Ambiguous column name 'Name'".

PS. All three tables [MyTable1], [MyTable2], [MyTable3] are a product of a previous selection.

回答1:

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.

回答2:

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]

回答3:

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

来源：https://stackoverflow.com/questions/6712127/select-countdistinct-name-from-several-tables

标签

sql

sql-server

select

count

distinct