Question

I am newbie to SQL Joins.
I have two tables

Version

Vid, VName, IsActive
1    V1     1
2    V2     0
3    V3     1

Sub-Version

SVid,Vid,  VName
1     1     0.1
2     1     0.2
3     2     0.1

In above tables each Version has many Sub-Version’s .

I need to fetch results from the above tables where the output should be like this.

Vid, VName, IsActive, SubVersionExists(Bit)
1    V1      1        1
2    V2      0        1
3    V3      1        0

in above result set the column name “SubVersionExists” represents if the version has SubVersion records in Sub-Version table.

Hope this explains the problem scenario well.

Thanks in advance.

Answer 1

Here is a version without joins, but using EXISTS (SELECT ... ) which returns a boolean value:

http://sqlfiddle.com/#!5/712bd/9

SELECT
  Version.Vid, Version.VName, Version.IsActive,
  EXISTS (
    SELECT NULL FROM SubVersion WHERE SubVersion.Vid = Version.Vid
  ) AS SubVersionExists
FROM Version;

Or, if your sql engine doesn’t convert booleans to 0/1, you can use CASE:

SELECT
  Version.Vid, Version.VName, Version.IsActive,
  CASE WHEN
    EXISTS (SELECT NULL FROM SubVersion WHERE SubVersion.Vid = Version.Vid)
  THEN 1 ELSE 0 END   AS SubVersionExists
FROM Version

Another version with LEFT JOIN + GROUP BY, if you don’t want to use COUNT():

SELECT
  Version.Vid, Version.VName, Version.IsActive,
  CASE WHEN SubVersionVids.Vid IS NOT NULL
       THEN 1 ELSE 0 END   AS SubVersionExists
FROM      Version
LEFT JOIN (
  SELECT   SubVersion.Vid
  FROM     SubVersion
  GROUP BY SubVersion.Vid
) AS SubVersionVids
  ON SubVersionVids.Vid = Version.Vid;

Or you can use DISTINCT instead of GROUP BY:

SELECT
  Version.Vid, Version.VName, Version.IsActive,
  CASE WHEN SubVersionVids.Vid IS NOT NULL
       THEN 1 ELSE 0 END   AS SubVersionExists
FROM      Version
LEFT JOIN (
  SELECT DISTINCT SubVersion.Vid
  FROM            SubVersion
) AS SubVersionVids
  ON SubVersionVids.Vid = Version.Vid;