Logical AND between table elements in T-SQL - sql

Question

I have n tables all with the same fields: Username and Value. The same Username can have multiple registers on each table but the combination Username/Value is unique on each one.
I want to join the tables into a single one which contains all the users who appear on all the tables with all the different (Username/Value) pairs.
Example
Table A: {(User1,Value1);(User1,Value2);(User2,Value2);(User3,Value4)]
Table B: {(User1,Value4);(User3,Value5)]
Table C: {(User1,Value5);(User1,Value2);(User2,Value7);(User3,Value8)]
Desired output
Table D: {(User1,Value1);(User1,Value2);(User1,Value4);(User1,Value5);(User3,Value4);(User3,Value5);(User3,Value8)}
Now I'm doing multiple joins (using perl) like this
SELECT *
INTO $target_table
FROM (SELECT *
FROM $table1
WHERE bname IN (SELECT DISTINCT bname FROM $table2)
UNION
SELECT *
FROM $table2
WHERE bname IN (SELECT DISTINCT bname FROM $table1)
) UN
and then doing the same join between a third table and target_table and so on, but I think it should be a better way.
Any hints?

Answer 1

You can use UNION for this:
SELECT username, value
FROM $table1
UNION
SELECT username, value
FROM $table2
...
SELECT username, value
FROM $tablex
SQL Fiddle Demo
This will return you distinct records. If you are interested in duplicates, use UNION ALL.
Given your edits, it appears you only want to return records if the user is in all the tables.
Breaking that down, you need to do a few things. First, combine all your records together again, but this time denote which table each are coming from. Then you need to know the count of tables each user is in. Finally you need to check that number against the overall number of tables.
Here's one way using a few CTEs:
WITH CTE AS (
SELECT username, value, 1 AS tbl
FROM t1
UNION
SELECT username, value, 2 AS tbl
FROM t2
UNION
SELECT username, value, 3 AS tbl
FROM t3
),
CTECnt AS (
SELECT username, COUNT(DISTINCT tbl) tblCnt
FROM CTE
GROUP BY username
),
CTEMaxCnt AS (
SELECT COUNT(DISTINCT tbl) MaxCnt
FROM CTE
)
SELECT C.username, C.value
FROM CTE C
JOIN CTECnt C2 ON C.username = C2.username
JOIN CTEMaxCnt C3 ON C2.tblCnt = C3.MaxCnt
Another SQL Fiddle Demo

Answer 2

With Combined As
(
Select 'A' As TableName, Username, Value
From TableA
Union All
Select 'B', Username, Value
From TableB
Union All
Select 'C', Username, Value
From TableC
)
Select C.Username, C.Value
From Combined As C
Join (
Select C1.Username
From Combined As C1
Group By C1.Username
Having Count(Distinct C1.TableName) = 3
) As Z
On Z.Username = C.Username
Group By C.Username, C.Value
SQL Fiddle version