Is it possible to remove the cursor from this dynamic query? - sql

Question

Simplified example of what I have:
Two tables (table_1 and table_2) which have one similar column (Id) but also several "payload" columns with different names (col_1_1, col_2_1, col_2_2). Amount of "payload" columns is different for different tables.
I am interested in extracting the IDs from both tables into another table for rows which have all "payload" columns empty.
There is a list of all "payload" columns for all tables which can be used (#temp)
This is how it is done with cursor:
CREATE TABLE #temp (tab nvarchar(20) not null, col nvarchar(20) not null)
INSERT INTO #temp SELECT 'table_1','col_1_1' UNION SELECT 'table_2','col_2_1' UNION SELECT 'table_2','col_2_2'
DECLARE #table_name nvarchar(20)
DECLARE #sql nvarchar(max)
DECLARE curs CURSOR FOR (SELECT DISTINCT tab FROM #temp)
OPEN curs
FETCH NEXT FROM curs INTO #table_name
WHILE ##FETCH_STATUS = 0
BEGIN
SELECT #sql = ISNULL(#sql,'')+col+' IS NULL AND ' FROM #temp WHERE tab = #table_name
SET #sql += 'Id IS NOT NULL'
SET #sql = 'INSERT INTO #temp_master SELECT ID FROM '+#table_name+' WHERE '+#sql
print #sql
SET #sql = ''
FETCH NEXT FROM curs INTO #table_name
END
CLOSE curs
DEALLOCATE curs
This is the result:
INSERT INTO #temp_master SELECT ID FROM table_1 WHERE col_1_1 IS NULL AND Id IS NOT NULL
INSERT INTO #temp_master SELECT ID FROM table_2 WHERE col_2_1 IS NULL AND col_2_2 IS NULL AND Id IS NOT NULL
Is it possible to remove the cursor to get the same resulting dynamic query? The problem is that I am unable to have dynamic "IS NULL AND" part for different tables when I remove the cursor.

Answer 1

It's possible to get rid of that cursor. This is probably what you need:
CREATE TABLE #temp (tab nvarchar(20) not null, col nvarchar(20) not null)
INSERT INTO #temp SELECT 'table_1','col_1_1' UNION SELECT 'table_2','col_2_1' UNION SELECT 'table_2','col_2_2'
DECLARE #table_name nvarchar(20)
DECLARE #sql nvarchar(max) = ''
select #sql = 'INSERT INTO #temp_master SELECT ID FROM ' + t.tab + ' WHERE Id IS NOT NULL AND ' + substring(t.cols, 0, len(t.cols)-3) + '
' + #sql from
(
SELECT
distinct
t2.tab,
stuff(
(
select t1.col + cast(' IS NULL AND ' as varchar(max))
from #temp t1
WHERE t1.tab = t2.tab
order by t1.tab
for xml path('')
), 1, 0, '') AS cols
FROM
#temp t2
) as t
order by t.tab desc
print #sql
drop table #temp

Answer 2

That is a regular CONCAT question, you can find many approaches to accomplish it without cursor. One of approaches is a cursor by the way and it's not that bad for such a task.
Another and more popular - FOR XML which can guarantee row order if any defined:
DECLARE #sql VARCHAR(MAX)
CREATE TABLE #temp (tab nvarchar(20) not null, col nvarchar(20) not null)
INSERT INTO #temp SELECT 'table_1','col_1_1' UNION SELECT 'table_2','col_2_1' UNION SELECT 'table_2','col_2_2'
SET #sql = (SELECT (
SELECT '
INSERT INTO #temp_master (ID) SELECT t.ID FROM '+t.tab +' t WHERE t.Id IS NOT NULL'
+ (select ' AND t.' + tt.col + ' is NULL' from #temp tt WHERE tt.tab = t.tab FOR XML PATH(''), TYPE).value('.', 'varchar(max)')
FROM #temp t
GROUP BY t.tab
FOR XML PATH(''), TYPE).value('.', 'varchar(max)'))
PRINT #sql
DROP TABLE #temp
A little "tricky" thing is that you have two things to collaps:
many queries (separate for specific table)
many columns per table
So you there is one inner FOR XML to collapse columns per table and another - to combine all queries into one big script.