Creating tables based off of SchemaColumn & SchemaTable composite table - sql-server

Question

I wanted to create a new set of tables in a db using a stored procedure and a build table.
The build table includes the following columns and some sample rows:
tblNm colNm colTyp colLen colReq colWarning colUni colComUni
account personID Decimal NULL 0 0 0 0
account studentNum String 15 0 0 0 0
I was considering using multiple cursors as a form of nested looping, but I cannot figure out how to define the column parameters in the nested procedure because cursors only return one value.
I am considering to build an alter statement that parses these values. How could I do this?

Answer 1

You can solve the problem by using two cursors or one cursor. Two cursors will make the code more readable. One cursor will be more efficient.
Two cursors
The code below demonstrates how to use two cursors to iterate through the tables and columns.
DECLARE #tblNm VARCHAR(MAX)
DECLARE cTables CURSOR FOR
SELECT tblNm
FROM CompositeSchema
GROUP BY tblNm
ORDER BY tblNm
OPEN cTables
FETCH cTables INTO #tblNm
WHILE ##FETCH_STATUS=0
BEGIN
PRINT 'Processing table '+#tblNm
-- Start of code to execute for each table
DECLARE #sqlCreateTable VARCHAR(MAX)
SET #sqlCreateTable = 'CREATE TABLE ['+#tblNm+'] ('
DECLARE #colNm VARCHAR(MAX),#colTyp VARCHAR(MAX),#colLen INT,#colReq BIT,#colWarning BIT,#colUni BIT,#colComUni BIT
DECLARE #isFirst BIT
SET #isFirst = 1
DECLARE cCols CURSOR FOR
SELECT colNm,colTyp,colLen,colReq,colWarning,colUni,colComUni
FROM CompositeSchema
WHERE tblNm=#tblNm
ORDER BY colComUni DESC,colNm ASC
OPEN cCols
FETCH cCols INTO #colNm,#colTyp,#colLen,#colReq,#colWarning,#colUni,#colComUni
WHILE ##FETCH_STATUS=0
BEGIN
PRINT 'Processing column ['+#tblNm+'].['+#colNm+']'
-- Start of code to process each column (simplified!)
IF #isFirst=0
SET #sqlCreateTable = #sqlCreateTable+','
SET #isFirst = 0
SET #sqlCreateTable = #sqlCreateTable+'['+#colNm+'] '+#colTyp
IF NOT #colLen IS NULL
SET #sqlCreateTable = #sqlCreateTable+'('+CAST(#colLen AS VARCHAR)+')'
-- End of code to process each column
FETCH cCols INTO #colNm,#colTyp,#colLen,#colReq,#colWarning,#colUni,#colComUni
END
CLOSE cCols
DEALLOCATE cCols
SET #sqlCreateTable = #sqlCreateTable+')'
PRINT #sqlCreateTable
-- EXEC(#sqlCreateTable)
-- End of code to execute for each table
FETCH cTables INTO #tblNm
END
CLOSE cTables
DEALLOCATE cTables
One cursor
In this case we use just one cursor. We keep track of what the current table is that we are processing in the #currentTblNm variable. Whenever the variable changes, we create all columns at once.
DECLARE #currentTblNm VARCHAR(MAX),#sqlCreateTable VARCHAR(MAX)
SET #currentTblNm = ''
DECLARE #tblNm VARCHAR(MAX),#colNm VARCHAR(MAX),#colTyp VARCHAR(MAX),#colLen INT,#colReq BIT,#colWarning BIT,#colUni BIT,#colComUni BIT
DECLARE #isFirst BIT
SET #isFirst = 1
DECLARE cCols CURSOR FOR
SELECT tblNm,colNm,colTyp,colLen,colReq,colWarning,colUni,colComUni
FROM CompositeSchema
ORDER BY tblNm ASC,colComUni DESC,colNm ASC
OPEN cCols
FETCH cCols INTO #tblNm,#colNm,#colTyp,#colLen,#colReq,#colWarning,#colUni,#colComUni
WHILE ##FETCH_STATUS=0
BEGIN
IF #currentTblNm<>#tblNm
BEGIN
IF #sqlCreateTable<>''
BEGIN
SET #sqlCreateTable = #sqlCreateTable+')'
PRINT #sqlCreateTable
--EXEC (#sqlCreateTable)
END
SET #isFirst = 1
SET #sqlCreateTable = 'CREATE TABLE ['+#tblNm+'] ('
SET #currentTblNm = #tblNm
PRINT 'Processing table ['+#tblNm+']'
END
-- Start of code to process each column (simplified!)
IF #isFirst=0
SET #sqlCreateTable = #sqlCreateTable+','
SET #isFirst = 0
SET #sqlCreateTable = #sqlCreateTable+'['+#colNm+'] '+#colTyp
IF NOT #colLen IS NULL
SET #sqlCreateTable = #sqlCreateTable+'('+CAST(#colLen AS VARCHAR)+')'
-- End of code to process each column
FETCH cCols INTO #tblNm,#colNm,#colTyp,#colLen,#colReq,#colWarning,#colUni,#colComUni
END
CLOSE cCols
DEALLOCATE cCols
IF #sqlCreateTable<>''
BEGIN
SET #sqlCreateTable = #sqlCreateTable+')'
PRINT #sqlCreateTable
-- EXEC(#sqlCreateTable)
END
Both pieces of code, Two cursors and One cursor are simplified. The logic to properly create all of the constraints (like primary key, unique constraints, foreign keys etc.), the logic to properly map the column data types, and not to forget, the making the distinction between creating a new table and altering an existing table is beyond the scope of this post.
Worth mentioning is that you could also use declarative SQL code with FOR XML to create the table structure. This is possible and would be able to generate the CREATE TABLE statements with a much better performance. From experience I know that this code will be much harder to maintain and you might run into the limitations of declarative SQL.