Select distinct from multiple fields using sql

Question

I have 5 columns corresponding to answers in a trivia game database - right, wrong1, wrong2, wrong3, wrong4

I want to return all possible answers without duplicates.

Answer 1

Is this what you wanted?

select distinct c1 from t

Answer 2

SELECT DISTINCT(ans) FROM (
    SELECT right AS ans FROM answers
    UNION
    SELECT wrong1 AS ans FROM answers
    UNION
    SELECT wrong2 AS ans FROM answers
    UNION
    SELECT wrong3 AS ans FROM answers
    UNION
    SELECT wrong4 AS ans FROM answers
) AS Temp

The DISTINCT is superfluous, because the UNION will not return rows that are identical for all columns. (When you want duplicated, or if you know that no duplicates exist, use UNION ALL for faster performance)

This will give you a single list containing all the answers. You'll still have duplicates though if you have multiple copies of the same answer within a single column.

That should not be the case if you use UNION, only if you use UNION ALL

SELECT [value] INTO #TEMP
FROM
(
SELECT  [value] = 1
UNION ALL SELECT 2
UNION ALL SELECT 3
UNION ALL SELECT 1
) AS X

(4 row(s) affected)

SELECT [value] 
FROM    #TEMP

value       
----------- 
1
2
3
1

(4 row(s) affected)

SELECT [value] 
FROM    #TEMP
UNION 
SELECT [value]
FROM    #TEMP

value       
----------- 
1
2
3

(3 row(s) affected)

Answer 3

In MySQL AND MS SQL:

SELECT
      CASE
        WHEN which = 1 THEN c1
        WHEN which = 2 THEN c2
        WHEN which = 3 THEN c3
        WHEN which = 4 THEN c4
        WHEN which = 5 THEN c5
      END AS answer,
      which
FROM mytable, (
     SELECT 1 AS which
     UNION ALL 
     SELECT 2
     UNION ALL 
     SELECT 3
     UNION ALL 
     SELECT 4
     UNION ALL 
     SELECT 5
) w

For Oracle, add FROM DUAL to each of the number selects.

Answer 4

I provided one answer above.

However I figured a much better way to do it using UNPIVOT.

SELECT DISTINCT(ans)
FROM (
    SELECT [Name], ANS 
    FROM (
        SELECT right, wrong1, wrong2, wrong3, wrong4 
        FROM answers
    ) AS PVT
    UNPIVOT 
    (ans FOR [Name] IN (right, wrong1, wrong2, wrong3, wrong4)) AS UNPVT
) AS OUTPUT;

You can provide any WHERE clause in the internal subquery:

SELECT DISTINCT(ans)
FROM (
    SELECT [Name], ANS 
    FROM (
        SELECT right, wrong1, wrong2, wrong3, wrong4 
        FROM answers
        WHERE (...)
    ) AS PVT
    UNPIVOT 
    (ans FOR [Name] IN (right, wrong1, wrong2, wrong3, wrong4)) AS UNPVT
) AS OUTPUT;

Answer 5

This should give you all distinct values from the table. I presume you'd want to add where clauses to select only for a particular question. However, this solution requires 5 subqueries and can be slow if your table is huge.

SELECT DISTINCT(ans) FROM (
    SELECT right AS ans FROM answers
    UNION
    SELECT wrong1 AS ans FROM answers
    UNION
    SELECT wrong2 AS ans FROM answers
    UNION
    SELECT wrong3 AS ans FROM answers
    UNION
    SELECT wrong4 AS ans FROM answers
) AS Temp

Answer 6

This is the exact answer.

SELECT (ans) FROM (
    SELECT correct AS ans FROM tGeoQuiz 
    UNION
    SELECT wrong1 AS ans FROM tGeoQuiz 
    UNION
    SELECT wrong2 AS ans FROM tGeoQuiz
    UNION
    SELECT wrong3 AS ans FROM tGeoQuiz
    UNION
    SELECT wrong4 AS ans FROM tGeoQuiz 
) AS Temp