How To Insert Multiple Rows From A Single Row? (SQL Server)

Introduction

When working with data in SQL Server, there are situations where you need to insert multiple rows from a single row in another table. This can be achieved using various techniques, including using a loop, using a recursive CTE, or using a set-based approach. In this article, we will explore how to insert multiple rows from a single row in SQL Server.

Problem Statement

Let's consider a scenario where we have two tables, Table1 and Table2. Table1 has a [Qty] field, and we want to copy every row from Table1 into Table2 each row copied [Qty] times. For example, if Table1 has the following data:

We want to insert the following data into Table2:

Method 1: Using a Loop

One way to achieve this is by using a loop to insert the rows into Table2. We can use a cursor to iterate over the rows in Table1 and insert each row [Qty] times into Table2.

DECLARE @ID INT, @Name VARCHAR(50), @Qty INT

DECLARE cur CURSOR FOR
SELECT ID, Name, Qty
FROM Table1

OPEN cur

FETCH NEXT FROM cur INTO @ID, @Name, @Qty

WHILE @@FETCH_STATUS = 0
BEGIN
    DECLARE @i INT = 1

    WHILE @i <= @Qty
    BEGIN
        INSERT INTO Table2 (ID, Name, Qty)
        VALUES (@ID, @Name, @i)

        SET @i = @i + 1
    END

    FETCH NEXT FROM cur INTO @ID, @Name, @Qty
END

CLOSE cur
DEALLOCATE cur

Method 2: Using a Recursive CTE

Another way to achieve this is by using a recursive CTE to insert the rows into Table2. We can use a recursive CTE to generate the rows to be inserted into Table2.

WITH RecursiveCTE AS (
    SELECT ID, Name, Qty, 1 AS RowNumber
    FROM Table1
    UNION ALL
    SELECT ID, Name, Qty, RowNumber + 1
    FROM RecursiveCTE
    WHERE RowNumber < Qty
)
INSERT INTO Table2 (ID, Name, Qty)
SELECT ID, Name, RowNumber
FROM RecursiveCTE
OPTION (MAXRECURSION 0)

Method 3: Using a Set-Based Approach

A set-based approach is generally more efficient than a loop-based approach. We can use a set-based approach to insert the rows into Table2 by using a SELECT statement with a CROSS JOIN to generate the rows to be inserted into Table2.

INSERT INTO Table2 (ID, Name, Qty)
SELECT ID, Name, i
FROM Table1
CROSS JOIN (SELECT 1 AS i UNION ALL SELECT 2 UNION ALL SELECT 3) AS Numbers
WHERE Numbers.i <= Qty

Conclusion

In this article, we have explored three methods to insert multiple rows from a single row in SQL Server. We have used a loop, a recursive CTE, and a set-based approach to achieve this. The set-based approach is generally more efficient than the other two methods. We have also provided sample data and code to demonstrate each method.

Best Practices

When working with data in SQL Server, it is essential to follow best practices to ensure data integrity and performance. Here are some best practices to keep in mind:

• Use a set-based approach instead of a loop-based approach.
• Use a recursive CTE instead of a loop-based approach when dealing with hierarchical data.
• Use a SELECT statement with a CROSS JOIN to generate the rows to be inserted into Table2.
• Use a UNION ALL operator to combine the results of multiple SELECT statements.
• Use a OPTION (MAXRECURSION 0) clause to specify the maximum recursion depth.

Q: What is the best method to insert multiple rows from a single row in SQL Server?

A: The best method to insert multiple rows from a single row in SQL Server is to use a set-based approach. This approach is generally more efficient than a loop-based approach and can handle large datasets.

Q: How do I use a set-based approach to insert multiple rows from a single row in SQL Server?

A: To use a set-based approach, you can use a SELECT statement with a CROSS JOIN to generate the rows to be inserted into Table2. For example:

INSERT INTO Table2 (ID, Name, Qty)
SELECT ID, Name, i
FROM Table1
CROSS JOIN (SELECT 1 AS i UNION ALL SELECT 2 UNION ALL SELECT 3) AS Numbers
WHERE Numbers.i <= Qty

Q: What is the difference between a recursive CTE and a loop-based approach?

A: A recursive CTE is a set-based approach that uses a recursive query to generate the rows to be inserted into Table2. A loop-based approach, on the other hand, uses a cursor to iterate over the rows in Table1 and insert each row [Qty] times into Table2.

Q: How do I use a recursive CTE to insert multiple rows from a single row in SQL Server?

A: To use a recursive CTE, you can use the following query:

WITH RecursiveCTE AS (
    SELECT ID, Name, Qty, 1 AS RowNumber
    FROM Table1
    UNION ALL
    SELECT ID, Name, Qty, RowNumber + 1
    FROM RecursiveCTE
    WHERE RowNumber < Qty
)
INSERT INTO Table2 (ID, Name, Qty)
SELECT ID, Name, RowNumber
FROM RecursiveCTE
OPTION (MAXRECURSION 0)

Q: What is the maximum recursion depth in a recursive CTE?

A: The maximum recursion depth in a recursive CTE is specified by the OPTION (MAXRECURSION 0) clause. This clause allows the query to recurse up to the maximum allowed depth, which is 100 by default.

Q: How do I handle errors when inserting multiple rows from a single row in SQL Server?

A: To handle errors when inserting multiple rows from a single row in SQL Server, you can use a TRY-CATCH block to catch any errors that occur during the insertion process. For example:

BEGIN TRY
    INSERT INTO Table2 (ID, Name, Qty)
    SELECT ID, Name, i
    FROM Table1
    CROSS JOIN (SELECT 1 AS i UNION ALL SELECT 2 UNION ALL SELECT 3) AS Numbers
    WHERE Numbers.i <= Qty
END TRY
BEGIN CATCH
    PRINT 'Error inserting rows: ' + ERROR_MESSAGE()
END CATCH

Q: How do I optimize the performance of inserting multiple rows from a single row in SQL Server?

A: To optimize the performance of inserting multiple rows from a single row in SQL Server, you can use the following techniques:

• Use a set-based approach instead of a loop-based approach.
• Use a recursive CTE instead of a loop-based approach when dealing with hierarchical data.
• Use a SELECT statement with a CROSS JOIN to generate the rows to be inserted into Table2.
• Use a UNION ALL operator to combine the results of multiple SELECT statements.
• Use a OPTION (MAXRECURSION 0) clause to specify the maximum recursion depth.

By following these techniques, you can optimize the performance of inserting multiple rows from a single row in SQL Server.