Using loop with UNION t-sql - sql-server

Question

I have a need to query several tables, with similar data, but I don't want to write a lengthy UNION statement. Also, the number of tables can vary. Currently I accomplish this using Coldfusion by:
looping over a list of tablenames
performing the UNION inside the loop
Like this
set i = 0
set table_suffixes = "blue,green,red,yellow"
loop list="table_suffixes" index="idx_suffix"
set tName = "table_" & idx_suffix
if i > 0
UNION
end if
select *
FROM tName
end loop
Can this be done in T-SQL? I have not been able to find any articles about using a list to control a loop. The closest I could find would be to place a query in the loop. Like This
While (SELECT table_suffix FROM vendor_name WHERE discipline = 'electrical')
BEGIN
*union query...*
END
Any help to point me in the right direction is much appreciated. I am using SQL-Server 2008

Answer 1

Although generally nor recommended, here is an example of generating and executing dynamic SQL within T-SQL.
Note I have used UNION ALL. You should understand the differences between this and UNION
Once you have this working you could wrap it in a a stored procedure that accepts a parameter to use for DISCIPLINE
Also please note, the fact you have to do this implies that you have some serious design flaws in your database.
DECLARE #TableName VARCHAR(100)
DECLARE #DSQL VARCHAR(4000)
SET #DSQL = ''
DECLARE cTableList CURSOR FOR
SELECT table_suffix FROM vendor_name WHERE discipline = 'electrical'
OPEN cTableList
FETCH NEXT FROM cTableList INTO #TableName
WHILE ##FETCH_STATUS = 0
BEGIN
SET #DSQL = #DSQL + 'SELECT * FROM ' + #TableName + ' UNION ALL '
END
CLOSE cTableList
DEALLOCATE cTableList
-- Remove the last UNION ALL
IF LEN(#DSQL) > 11 SET #DSQL = LEFT(#DSQL,LEN(#DSQL) - 11)
-- Print it out for debugging purposes
PRINT (#DSQL)
-- Execute it
EXEC (#DSQL)

Answer 2

Try this one -
Query:
DECLARE #SQL NVARCHAR(MAX)
SELECT #SQL = STUFF((
SELECT CHAR(13) + 'UNION ALL' + CHAR(13) + 'SELECT * FROM [' + s.name + '].[' + o.name + ']'
FROM sys.objects o
JOIN sys.schemas s ON o.[schema_id] = s.[schema_id]
WHERE o.[type] = 'U'
FOR XML PATH(''), TYPE).value('.', 'NVARCHAR(MAX)'), 1, 11, '')
PRINT #SQL
Output:
SELECT * FROM [dbo].[test1]
UNION ALL
SELECT * FROM [dbo].[MyTable]
UNION ALL
...
In your case see something like this -
DECLARE #SQL NVARCHAR(MAX)
SELECT #SQL = STUFF((
SELECT CHAR(13) + 'UNION ALL' + CHAR(13) + 'SELECT * FROM ' + table_suffix
FROM vendor_name
WHERE discipline = 'electrical'
WHERE o.[type] = 'U'
FOR XML PATH(''), TYPE).value('.', 'NVARCHAR(MAX)'), 1, 11, '')
PRINT #SQL