Select unique column names from a list of joined tables - sql

Question

I have a list of tables that can be joined together by the same PK column. Since this list of tables can vary from project to project, I want to create a query that can be dynamic enough to pull all unique columns from these tables.
For example, I have three tables below:
Table A (PK field, column1, column 2)
Table B (PK field, column3, column 4)
Table C (PK field, column5, column 5)
These three tables are joined on "PK field" column, and I want the query output to be something like:
PK field column1 column2 column3 column4 column5
..data.. ..data.. ..data.. ..data.. ..data.. ..data..
At the end, this query will be part of a SQL function or SP, so the user can define a list of tables, and PK field at the beginning, then executing it shall return my expected output with dataset.
I think about use this query below but the result is not what I like:
SELECT COLUMN_NAME FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_NAME = ''
Any advice about how I should design this SP or function, will be appreciated.
Thanks in advance.
DDL for two example tables:
CREATE TABLE [dbo].[G_bDEM](
[blaiseKey_code] [nvarchar](255) NULL,
[qSex] [int] NULL,
[qDOB] [datetime] NULL,
[qDOBNR] [int] NULL,
[qAge] [int] NULL,
[qAgeNR] [int] NULL,
[qAgeRange] [int] NULL,
[qAge15OrOver] [int] NULL,
[qNotEligible] [nvarchar](1) NULL,
[qBornInNZ] [int] NULL,
[qCountryOfBirth] [nvarchar](2) NULL,
[qArriveNZYr] [int] NULL,
[qArriveNZYrNR] [int] NULL,
[qArriveNZMth] [int] NULL,
[bDEM_BOP_qHowManyRaised] [int] NULL,
[bDEM_BOP_q1stParentBornNZ] [int] NULL,
[bDEM_BOP_q2ndParentBornNZ] [int] NULL,
[bDEM_BOP_qHowManyParentBornNZ] [int] NULL,
[qMaoriDescent] [int] NULL,
[qSchQual] [int] NULL,
[qSchQualOth] [nvarchar](200) NULL,
[qSchQualOthNR] [int] NULL,
[qSchQualYr] [int] NULL,
[qSchQualYrNR] [int] NULL,
[qPostSchQual] [int] NULL,
[q3MthsStudy] [int] NULL,
[qHighestQual] [int] NULL,
[qHighestQualOth] [nvarchar](200) NULL,
[qHighestQualOthNR] [int] NULL,
[qHighestQualYr] [int] NULL,
[qHighestQualYrNR] [int] NULL,
[qWorkIntro] [nvarchar](1) NULL,
[qDidPaidWork] [int] NULL,
[qAwayFromWork] [int] NULL,
[qFamilyBusWork] [int] NULL,
[bDEM_WOR_qPaidWorkIntro] [nvarchar](1) NULL,
[bDEM_WOR_qJobsNum] [int] NULL,
[bDEM_WOR_qJobsNumNR] [int] NULL,
[bDEM_WOR_tabDEM_T2_fTotMins] [int] NULL,
[bDEM_WOR_q2JobsNoHrsIntro] [nvarchar](1) NULL,
[bDEM_WOR_q2Jobs2HrsIntro] [nvarchar](1) NULL,
[bDEM_WOR_q2Jobs1HrsIntro] [nvarchar](1) NULL,
[bDEM_WOR_qOccupation] [nvarchar](200) NULL,
[bDEM_WOR_qOccupationNR] [int] NULL,
[bDEM_WOR_qMainTasks] [nvarchar](200) NULL,
[bDEM_WOR_qMainTasksNR] [int] NULL,
[bDEM_WOR_qFeelAboutJob] [int] NULL,
[bDEM_WOR_qEmployArrangement] [int] NULL,
[bDEM_WOR_qPermEmployee] [int] NULL,
[qHasJobToStart] [int] NULL,
[qLookedForWork] [int] NULL,
[qJobSearchA] [int] NULL,
[qJobSearchB] [int] NULL,
[qJobSearchC] [int] NULL,
[qJobSearchD] [int] NULL,
[qJobSearchE] [int] NULL,
[qJobSearchF] [int] NULL,
[qJobSearchG] [int] NULL,
[qJobSearchH] [int] NULL,
[qJobSearchI] [int] NULL,
[qJobSearchOth] [nvarchar](200) NULL,
[qJobSearchOthNR] [int] NULL,
[qCouldStartLastWk] [int] NULL,
[qIncTotalAmt] [int] NULL,
[fCountryName] [nvarchar](60) NULL
) ON [PRIMARY]
GO
CREATE TABLE [dbo].[G_bLWW](
[blaiseKey_code] [nvarchar](255) NULL,
[qThingsWorthwhileScale] [int] NULL
) ON [PRIMARY]

Answer 1

This script generate dynamic SQL for any table with similar PK name.
Query:
SET NOCOUNT ON
IF OBJECT_ID (N'dbo.A') IS NOT NULL
DROP TABLE dbo.A
IF OBJECT_ID (N'dbo.B') IS NOT NULL
DROP TABLE dbo.B
IF OBJECT_ID (N'dbo.C') IS NOT NULL
DROP TABLE dbo.C
CREATE TABLE dbo.A (PK_field INT PRIMARY KEY, column1 INT, column2 INT)
CREATE TABLE dbo.B (PK_field INT PRIMARY KEY, column3 INT, column4 INT)
CREATE TABLE dbo.C (PK_field INT PRIMARY KEY, column5 INT, [column 6] INT)
INSERT INTO dbo.A (PK_field, column1, column2)
VALUES (1, 1, 2), (2, 1, 2)
INSERT INTO dbo.B (PK_field, column3, column4)
VALUES (2, 3, 4)
INSERT INTO dbo.C (PK_field, column5, [column 6])
VALUES (1, 5, 6), (3, 5, 6)
DECLARE #SQL NVARCHAR(MAX)
;WITH cte AS
(
SELECT
column_name = '[' + c.name + ']'
, table_name = '[' + s.name + '].[' + o.name + ']'
FROM sys.columns c WITH (NOLOCK)
JOIN sys.objects o WITH (NOLOCK) ON c.[object_id] = o.[object_id]
JOIN sys.schemas s WITH (NOLOCK) ON o.[schema_id] = s.[schema_id]
WHERE o.name IN ('A', 'B', 'C')
AND s.name = 'dbo'
AND o.[type] = 'U'
), unicol AS (
SELECT TOP 1 column_name
FROM cte
GROUP BY cte.column_name
HAVING COUNT(cte.column_name) > 1
), cols AS
(
SELECT DISTINCT column_name
FROM cte
), tbl AS
(
SELECT DISTINCT table_name
FROM cte
), rs AS
(
SELECT
tbl.table_name
, column_name = ISNULL(cte.column_name, cols.column_name + ' = NULL')
FROM cols
CROSS JOIN tbl
LEFT JOIN cte ON cols.column_name = cte.column_name AND cte.table_name = tbl.table_name
), rs2 AS (
SELECT uni = ' UNION ALL' + CHAR(13) + 'SELECT ' + STUFF((
SELECT ', ' + rs.column_name
FROM rs
WHERE tbl.table_name = rs.table_name
GROUP BY rs.column_name
ORDER BY rs.column_name
FOR XML PATH(''), TYPE).value('.', 'NVARCHAR(MAX)'), 1, 2, '') +
' FROM ' + table_name
FROM tbl
)
SELECT #SQL = 'SELECT
' + STUFF((
SELECT CHAR(13) + ', ' + ISNULL(unicol.column_name, cols.column_name + ' = MAX(' + cols.column_name + ')')
FROM cols
LEFT JOIN unicol ON cols.column_name = unicol.column_name
FOR XML PATH(''), TYPE).value('.', 'NVARCHAR(MAX)'), 1, 2, ' ')
+ '
FROM
(' + STUFF((
SELECT CHAR(10) + uni
FROM rs2
FOR XML PATH(''), TYPE).value('.', 'NVARCHAR(MAX)'), 1, 11, '') + CHAR(13) +
') t
GROUP BY ' + (SELECT column_name FROM unicol)
PRINT #SQL
EXECUTE sys.sp_executesql #SQL
Output:
SELECT
[column 6] = MAX([column 6])
, [column1] = MAX([column1])
, [column2] = MAX([column2])
, [column3] = MAX([column3])
, [column4] = MAX([column4])
, [column5] = MAX([column5])
, [PK_field]
FROM (
SELECT [column 6] = NULL, [column1], [column2], [column3] = NULL, [column4] = NULL, [column5] = NULL, [PK_field] FROM [dbo].[A]
UNION ALL
SELECT [column 6] = NULL, [column1] = NULL, [column2] = NULL, [column3], [column4], [column5] = NULL, [PK_field] FROM [dbo].[B]
UNION ALL
SELECT [column 6], [column1] = NULL, [column2] = NULL, [column3] = NULL, [column4] = NULL, [column5], [PK_field] FROM [dbo].[C]
) t
GROUP BY [PK_field]
Results:
column 6 column1 column2 column3 column4 column5 PK_field
----------- ----------- ----------- ----------- ----------- ----------- -----------
6 1 2 NULL NULL 5 1
NULL 1 2 3 4 NULL 2
6 NULL NULL NULL NULL 5 3
Update in script:
DECLARE #SQL NVARCHAR(2000) -> NVARCHAR(MAX)
Output for your DDL:
SELECT
[blaiseKey_code]
, [bDEM_BOP_q1stParentBornNZ] = MAX([bDEM_BOP_q1stParentBornNZ])
, [bDEM_BOP_q2ndParentBornNZ] = MAX([bDEM_BOP_q2ndParentBornNZ])
, [bDEM_BOP_qHowManyParentBornNZ] = MAX([bDEM_BOP_qHowManyParentBornNZ])
, [bDEM_BOP_qHowManyRaised] = MAX([bDEM_BOP_qHowManyRaised])
, [bDEM_WOR_q2Jobs1HrsIntro] = MAX([bDEM_WOR_q2Jobs1HrsIntro])
, [bDEM_WOR_q2Jobs2HrsIntro] = MAX([bDEM_WOR_q2Jobs2HrsIntro])
, [bDEM_WOR_q2JobsNoHrsIntro] = MAX([bDEM_WOR_q2JobsNoHrsIntro])
, [bDEM_WOR_qEmployArrangement] = MAX([bDEM_WOR_qEmployArrangement])
, [bDEM_WOR_qFeelAboutJob] = MAX([bDEM_WOR_qFeelAboutJob])
, [bDEM_WOR_qJobsNum] = MAX([bDEM_WOR_qJobsNum])
, [bDEM_WOR_qJobsNumNR] = MAX([bDEM_WOR_qJobsNumNR])
, [bDEM_WOR_qMainTasks] = MAX([bDEM_WOR_qMainTasks])
, [bDEM_WOR_qMainTasksNR] = MAX([bDEM_WOR_qMainTasksNR])
, [bDEM_WOR_qOccupation] = MAX([bDEM_WOR_qOccupation])
, [bDEM_WOR_qOccupationNR] = MAX([bDEM_WOR_qOccupationNR])
, [bDEM_WOR_qPaidWorkIntro] = MAX([bDEM_WOR_qPaidWorkIntro])
, [bDEM_WOR_qPermEmployee] = MAX([bDEM_WOR_qPermEmployee])
, [bDEM_WOR_tabDEM_T2_fTotMins] = MAX([bDEM_WOR_tabDEM_T2_fTotMins])
, [fCountryName] = MAX([fCountryName])
, [q3MthsStudy] = MAX([q3MthsStudy])
, [qAge] = MAX([qAge])
, [qAge15OrOver] = MAX([qAge15OrOver])
, [qAgeNR] = MAX([qAgeNR])
, [qAgeRange] = MAX([qAgeRange])
, [qArriveNZMth] = MAX([qArriveNZMth])
, [qArriveNZYr] = MAX([qArriveNZYr])
, [qArriveNZYrNR] = MAX([qArriveNZYrNR])
, [qAwayFromWork] = MAX([qAwayFromWork])
, [qBornInNZ] = MAX([qBornInNZ])
, [qCouldStartLastWk] = MAX([qCouldStartLastWk])
, [qCountryOfBirth] = MAX([qCountryOfBirth])
, [qDidPaidWork] = MAX([qDidPaidWork])
, [qDOB] = MAX([qDOB])
, [qDOBNR] = MAX([qDOBNR])
, [qFamilyBusWork] = MAX([qFamilyBusWork])
, [qHasJobToStart] = MAX([qHasJobToStart])
, [qHighestQual] = MAX([qHighestQual])
, [qHighestQualOth] = MAX([qHighestQualOth])
, [qHighestQualOthNR] = MAX([qHighestQualOthNR])
, [qHighestQualYr] = MAX([qHighestQualYr])
, [qHighestQualYrNR] = MAX([qHighestQualYrNR])
, [qIncTotalAmt] = MAX([qIncTotalAmt])
, [qJobSearchA] = MAX([qJobSearchA])
, [qJobSearchB] = MAX([qJobSearchB])
, [qJobSearchC] = MAX([qJobSearchC])
, [qJobSearchD] = MAX([qJobSearchD])
, [qJobSearchE] = MAX([qJobSearchE])
, [qJobSearchF] = MAX([qJobSearchF])
, [qJobSearchG] = MAX([qJobSearchG])
, [qJobSearchH] = MAX([qJobSearchH])
, [qJobSearchI] = MAX([qJobSearchI])
, [qJobSearchOth] = MAX([qJobSearchOth])
, [qJobSearchOthNR] = MAX([qJobSearchOthNR])
, [qLookedForWork] = MAX([qLookedForWork])
, [qMaoriDescent] = MAX([qMaoriDescent])
, [qNotEligible] = MAX([qNotEligible])
, [qPostSchQual] = MAX([qPostSchQual])
, [qSchQual] = MAX([qSchQual])
, [qSchQualOth] = MAX([qSchQualOth])
, [qSchQualOthNR] = MAX([qSchQualOthNR])
, [qSchQualYr] = MAX([qSchQualYr])
, [qSchQualYrNR] = MAX([qSchQualYrNR])
, [qSex] = MAX([qSex])
, [qThingsWorthwhileScale] = MAX([qThingsWorthwhileScale])
, [qWorkIntro] = MAX([qWorkIntro])
FROM
(
SELECT [bDEM_BOP_q1stParentBornNZ], [bDEM_BOP_q2ndParentBornNZ], [bDEM_BOP_qHowManyParentBornNZ], [bDEM_BOP_qHowManyRaised], [bDEM_WOR_q2Jobs1HrsIntro], [bDEM_WOR_q2Jobs2HrsIntro], [bDEM_WOR_q2JobsNoHrsIntro], [bDEM_WOR_qEmployArrangement], [bDEM_WOR_qFeelAboutJob], [bDEM_WOR_qJobsNum], [bDEM_WOR_qJobsNumNR], [bDEM_WOR_qMainTasks], [bDEM_WOR_qMainTasksNR], [bDEM_WOR_qOccupation], [bDEM_WOR_qOccupationNR], [bDEM_WOR_qPaidWorkIntro], [bDEM_WOR_qPermEmployee], [bDEM_WOR_tabDEM_T2_fTotMins], [blaiseKey_code], [fCountryName], [q3MthsStudy], [qAge], [qAge15OrOver], [qAgeNR], [qAgeRange], [qArriveNZMth], [qArriveNZYr], [qArriveNZYrNR], [qAwayFromWork], [qBornInNZ], [qCouldStartLastWk], [qCountryOfBirth], [qDidPaidWork], [qDOB], [qDOBNR], [qFamilyBusWork], [qHasJobToStart], [qHighestQual], [qHighestQualOth], [qHighestQualOthNR], [qHighestQualYr], [qHighestQualYrNR], [qIncTotalAmt], [qJobSearchA], [qJobSearchB], [qJobSearchC], [qJobSearchD], [qJobSearchE], [qJobSearchF], [qJobSearchG], [qJobSearchH], [qJobSearchI], [qJobSearchOth], [qJobSearchOthNR], [qLookedForWork], [qMaoriDescent], [qNotEligible], [qPostSchQual], [qSchQual], [qSchQualOth], [qSchQualOthNR], [qSchQualYr], [qSchQualYrNR], [qSex], [qThingsWorthwhileScale] = NULL, [qWorkIntro] FROM [dbo].[G_bDEM]
UNION ALL
SELECT [bDEM_BOP_q1stParentBornNZ] = NULL, [bDEM_BOP_q2ndParentBornNZ] = NULL, [bDEM_BOP_qHowManyParentBornNZ] = NULL, [bDEM_BOP_qHowManyRaised] = NULL, [bDEM_WOR_q2Jobs1HrsIntro] = NULL, [bDEM_WOR_q2Jobs2HrsIntro] = NULL, [bDEM_WOR_q2JobsNoHrsIntro] = NULL, [bDEM_WOR_qEmployArrangement] = NULL, [bDEM_WOR_qFeelAboutJob] = NULL, [bDEM_WOR_qJobsNum] = NULL, [bDEM_WOR_qJobsNumNR] = NULL, [bDEM_WOR_qMainTasks] = NULL, [bDEM_WOR_qMainTasksNR] = NULL, [bDEM_WOR_qOccupation] = NULL, [bDEM_WOR_qOccupationNR] = NULL, [bDEM_WOR_qPaidWorkIntro] = NULL, [bDEM_WOR_qPermEmployee] = NULL, [bDEM_WOR_tabDEM_T2_fTotMins] = NULL, [blaiseKey_code], [fCountryName] = NULL, [q3MthsStudy] = NULL, [qAge] = NULL, [qAge15OrOver] = NULL, [qAgeNR] = NULL, [qAgeRange] = NULL, [qArriveNZMth] = NULL, [qArriveNZYr] = NULL, [qArriveNZYrNR] = NULL, [qAwayFromWork] = NULL, [qBornInNZ] = NULL, [qCouldStartLastWk] = NULL, [qCountryOfBirth] = NULL, [qDidPaidWork] = NULL, [qDOB] = NULL, [qDOBNR] = NULL, [qFamilyBusWork] = NULL, [qHasJobToStart] = NULL, [qHighestQual] = NULL, [qHighestQualOth] = NULL, [qHighestQualOthNR] = NULL, [qHighestQualYr] = NULL, [qHighestQualYrNR] = NULL, [qIncTotalAmt] = NULL, [qJobSearchA] = NULL, [qJobSearchB] = NULL, [qJobSearchC] = NULL, [qJobSearchD] = NULL, [qJobSearchE] = NULL, [qJobSearchF] = NULL, [qJobSearchG] = NULL, [qJobSearchH] = NULL, [qJobSearchI] = NULL, [qJobSearchOth] = NULL, [qJobSearchOthNR] = NULL, [qLookedForWork] = NULL, [qMaoriDescent] = NULL, [qNotEligible] = NULL, [qPostSchQual] = NULL, [qSchQual] = NULL, [qSchQualOth] = NULL, [qSchQualOthNR] = NULL, [qSchQualYr] = NULL, [qSchQualYrNR] = NULL, [qSex] = NULL, [qThingsWorthwhileScale], [qWorkIntro] = NULL FROM [dbo].[G_bLWW]
) t
GROUP BY [blaiseKey_code]

Answer 2

Try this :
DECLARE
#cols VARCHAR(MAX)
, #TableA VARCHAR(10)= 'TableA'
, #TableB VARCHAR(10)= 'TableB'
, #TableC VARCHAR(10)= 'TableC'
, #Pk VARCHAR(20)
SELECT
#cols = STUFF((
SELECT DISTINCT ', [' + c.column_name + ']'
FROM INFORMATION_SCHEMA.Columns c
WHERE c.table_name IN ( #TableA,#TableB,#TableC )
FOR XML PATH('')
), 1, 2, '');
SELECT #Pk = column_name
FROM INFORMATION_SCHEMA.KEY_COLUMN_USAGE
WHERE OBJECTPROPERTY(OBJECT_ID(constraint_name), 'IsPrimaryKey') = 1
AND table_name = #TableA
DECLARE #query VARCHAR(1000)
SET #query = 'SELECT ' + #cols + ' FROM ' + #TableA + ' JOIN ' + #TableB
+ ' ON ' + #TableA + '.' + #Pk + '=' + #TableB + '.' + #Pk
+ ' JOIN ' + #TableC + ' ON ' + #TableB + '.' + #Pk + '=' + #TableC
+ '.' + #Pk
EXEC (#query)
Do not forget the warning about special html characters in column name given by #Gordon.

Answer 3

You can only do this as a stored procedure. A SQL query returns a specified set of columns, no more, no less. The only way to get a variable number of columns is using dynamic SQL. And, functions don't support dynamic SQL.
You would need to construct a SQL statement, concatenating the column names from INFORMATION_SCHEMA.Columns. Something like this:
declare #cols varchar(max);
select #cols = stuff((select distinct ', ['+c.column_name+']'
from INFORMATION_SCHEMA.Columns c
where c.table_name in (<list of tables here>)
for xml path ('')
), 1, 2, '');
This will not work for column names that have special html characters, such as '<', '>', or '&'.
You can then construct the full query statement and execute it, with exec() or sp_executesql().
An alternative approach would be to create a view that has all the joins and all the columns. Let the SQL optimizer determine the best execution path.