How to avoid the cursor and what is the other way? - sql

Question

I am using the cursor in my stored procedure; I am hoping to remove the cursor from my SP. Please help me come up with a solution for how to avoid the cursor statement to normal update statement with dynamic.
Example Below:
Update Tablename set columnname(variable) = value from table A join Table B on A.condition = B.Condition where name = 'Test'(variable) and age = 18(variable)
Update Tablename set columnname(variable) = value from table A join Table B on A.condition = B.Condition where name = 'kumar'(variable) and age = 19(variable)
Update Tablename set columnname(variable) = value from table A join Table B on A.condition = B.Condition where name = 'babu'(variable) and age = 30(variable)
This is how my cursor will work. 300 Combination dynamically pick the data from table and update into the main table
I am trying to take out the cursor, and update statement should work similar to this, instead of writing 300 update statements, I want to write one update where all the 300 combinations should execute.
Below is my code which needs this solution:
BEGIN
DECLARE #Type VARCHAR(100)
DECLARE #TargetColumn VARCHAR(100)
DECLARE #SourceColumn VARCHAR(100)
DECLARE #SQL varchar(max)
DECLARE a_cursor CURSOR STATIC
FOR
SELECT [Type],[SourceColumn],[TargetColumn] FROM ref.tblEdsMap
GROUP BY [Type],[SourceColumn],[TargetColumn]
OPEN a_cursor
FETCH NEXT FROM a_cursor INTO #Type,#SourceColumn,#TargetColumn
WHILE ##FETCH_STATUS = 0
BEGIN
SET #SQL = 'UPDATE GCT SET GCT.' + #TargetColumn + ' = map.[TargetValue]
from EdsMap map
JOIN Table GCT
ON GCT.' + #SourceColumn + ' = map.[SourceValue]
where map.[Type]=''' + #Type + ''' and map.SourceColumn=''' + #SourceColumn+ ''''
Exec (#SQL)
PRINT #SQL
FETCH NEXT FROM a_cursor INTO #Type,#SourceColumn,#TargetColumn
END
CLOSE a_cursor
DEALLOCATE a_cursor
END

Answer 1

Rather than use an explicit cursor or a cursor cleverly disguised as a while loop, I prefer row concatenation operations for this type of problem.
DECLARE #cmd NVARCHAR(MAX) = N'';
SELECT #cmd += N'
UPDATE GCT
SET GCT.' + QUOTENAME(TargetColumn) + ' = map.TargetValue
FROM dbo.EdsMap AS map
INNER JOIN dbo.Table AS GCT
ON GCT.' + QUOTENAME(SourceColumn) + ' = map.SourceValue
WHERE map.[Type] = ''' + [Type] + '''
AND map.SourceColumn = ''' + [SourceColumn]+ ''';'
FROM ref.tblEdsMap
GROUP BY [Type], SourceColumn, TargetColumn;
EXEC sp_executesql #sql;

Answer 2

When I've done these in the past, I usually make up a transaction to encompass every update that's needed. Something like this:
CREATE TABLE #targets ([Type] VARCHAR(255),[SourceColumn] VARCHAR(255),[TargetColumn] VARCHAR(255));
INSERT INTO #targets
( [Type], [SourceColumn], [TargetColumn] )
SELECT [Type],[SourceColumn],[TargetColumn] FROM ref.tblEdsMap
GROUP BY [Type],[SourceColumn],[TargetColumn];
DECLARE #sql VARCHAR(MAX);
SET #sql = 'BEGIN TRAN' + CHAR(10) + CHAR(13);
SELECT #sql = #sql +
'UPDATE GCT SET GCT.' + [TargetColumn] + ' = map.[TargetValue]
from EdsMap map
JOIN Table GCT
ON GCT.' + [SourceColumn] + ' = map.[SourceValue]
where map.[Type]=''' + [Type] + ''' and map.SourceColumn=''' + [SourceColumn]+ ''';' + CHAR(10) + CHAR(13)
FROM #targets
SELECT #sql = #sql + 'COMMIT TRAN'
PRINT #sql
Exec (#SQL)
The update statements are still the same, i.e., you get one update per combination. But now you're running as one transaction batch. You could potentially be fancier with the dynamic SQL, so that you had just one update statement, but in my experience, it's too easy to get bad updates that way.
Doing it this way may not be any faster than a cursor. You'd have to test to be sure. With the examples where I've used this approach, it has generally been a faster approach.

Answer 3

Try using a table variable along with a WHILE loop instead, like so:
BEGIN
DECLARE #Type VARCHAR(100)
DECLARE #TargetColumn VARCHAR(100)
DECLARE #SourceColumn VARCHAR(100)
DECLARE #SQL varchar(max)
DECLARE #SomeTable TABLE
(
ID int IDENTITY (1, 1) PRIMARY KEY NOT NULL,
Type varchar(100),
SourceColumn varchar(100),
TargetColumn varchar(100)
)
DECLARE #Count int, #Max int
INSERT INTO #SomeTable (Type, SourceColumn, TargetColumn)
SELECT [Type],[SourceColumn],[TargetColumn]
FROM ref.tblEdsMap
GROUP BY [Type],[SourceColumn],[TargetColumn]
SELECT #Count = 1, #Max = COUNT(ID)
FROM #SomeTable
WHILE #Count <= #Max
BEGIN
SELECT
#Type = Type,
#SourceColumn = SourceColumn,
#TargetColumn = TargetColumn
FROM #SomeTable
WHERE ID = #Count
-- Your code
SET #SQL = 'UPDATE GCT SET GCT.' + #TargetColumn + ' = map.[TargetValue]
from EdsMap map
JOIN Table GCT
ON GCT.' + #SourceColumn + ' = map.[SourceValue]
where map.[Type]=''' + #Type + ''' and map.SourceColumn=''' + #SourceColumn+ ''''
Exec (#SQL)
PRINT #SQL
SET #Count = #Count + 1
END -- while
END