Question

I have 3 tables, each table will show records of users posting messages.

I have managed to sum the number of times each user has posted a message on each table and now I want to sum these 3 individual values together for each user.

Here is what I have managed so far:

Table USERMESSAGE:
SELECT U.SenderID, COUNT(U.SenderID) AS U_NUM
FROM USERMESSAGE AS U
WHERE U.SenderID != U.ReceiverID
GROUP BY U.SenderID

Table COMMENT:
SELECT C.UserID, COUNT(C.UserID) AS C_NUM
FROM COMMENT AS C
GROUP BY C.UserID

Table FRIENDLIST:
SELECT F.UserID, COUNT(F.UserID) AS F_NUM
FROM FRIENDLIST AS F
WHERE F.ListName = 'News Feed'
GROUP BY F.UserID

But not all user will post in all tables, so some UserID will not appear in some tables.

Answer 1

I would try something like this

DECLARE @Table1 TABLE(
        UserID INT
)

DECLARE @Table2 TABLE(
        UserID INT
)

DECLARE @Table3 TABLE(
        UserID INT
)

INSERT INTO @Table1 (UserID) SELECT 1
INSERT INTO @Table1 (UserID) SELECT 2
INSERT INTO @Table1 (UserID) SELECT 3

INSERT INTO @Table2 (UserID) SELECT 2
INSERT INTO @Table2 (UserID) SELECT 3

INSERT INTO @Table3 (UserID) SELECT 3

SELECT  ISNULL(CountTable1.CountUserIDTable1,0) +
        ISNULL(CountTable2.CountUserIDTable2,0) +
        ISNULL(CountTable3.CountUserIDTable3,0) CountOverTables,
        UserIDs.UserID
FROM    (
            SELECT  DISTINCT
                    UserID
            FROM    @Table1
            UNION   
            SELECT  DISTINCT
                    UserID
            FROM    @Table2
            UNION   
            SELECT  DISTINCT
                    UserID
            FROM    @Table3
        ) UserIDs LEFT JOIN
        (
            SELECT  UserID,
                    COUNT(UserID) CountUserIDTable1
            FROM    @Table1
            GROUP BY UserID
        ) CountTable1 ON UserIDs.UserID = CountTable1.UserID LEFT JOIN
        (
            SELECT  UserID,
                    COUNT(UserID) CountUserIDTable2
            FROM    @Table2
            GROUP BY UserID
        ) CountTable2 ON UserIDs.UserID = CountTable2.UserID LEFT JOIN
        (
            SELECT  UserID,
                    COUNT(UserID) CountUserIDTable3
            FROM    @Table3
            GROUP BY UserID
        ) CountTable3 ON UserIDs.UserID = CountTable3.UserID