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

Mar 1, 2025 by ADMIN 60 views

**How to Insert Multiple Rows from a Single Row in SQL Server**

Introduction

When working with SQL Server, you may encounter situations where you need to insert multiple rows from a single row in another table. This can be a common requirement in data migration, data transformation, or data analysis tasks. In this article, we will explore how to achieve this using SQL Server.

Understanding the Problem

Let's consider a scenario where you have a table called Table1 with a [Qty] column, and you want to copy every row from Table1 into another table called Table2, each row copied [Qty] times. For example, if Table1 contains the following data:

ID	Name	Qty
1	John	3
2	Jane	2

You want to insert the following data into Table2:

ID	Name	Qty
1	John	1
1	John	2
1	John	3
2	Jane	1
2	Jane	2

Using a Recursive Common Table Expression (CTE)

One way to achieve this is by using a recursive Common Table Expression (CTE). A CTE is a temporary result set that you can reference within a SELECT, INSERT, UPDATE, or DELETE statement.

Here's an example query that uses a recursive CTE to insert multiple rows from a single row in Table1 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, 1
FROM RecursiveCTE;

In this query, the CTE is defined with a recursive clause that selects the first row from Table1 and then recursively selects the next row until the RowNumber reaches the [Qty] value.

Using a Numbers Table

Another way to achieve this is by using a numbers table. A numbers table is a table that contains a sequence of numbers, which can be used to generate a sequence of rows.

Here's an example query that uses a numbers table to insert multiple rows from a single row in Table1 into Table2:

DECLARE @Qty INT = 1;

SELECT ID, Name, @Qty AS Qty
INTO #Numbers
FROM (VALUES (1)) AS T (ID);

WHILE @Qty < (SELECT MAX(Qty) FROM Table1)
BEGIN
  SET @Qty = @Qty + 1;
  INSERT INTO #Numbers (ID, Name, Qty)
  SELECT ID, Name, @Qty
  FROM Table1;
END;

INSERT INTO Table2 (ID, Name, Qty)
SELECT ID, Name, Qty
FROM #Numbers;

DROP TABLE #Numbers;

In this query, a numbers table is created with a single row containing the initial value of @Qty. Then, a while loop is used to insert additional rows into the numbers table until the maximum [Qty] value in Table1 is reached.

Using a Loop

Another way to achieve this is by using a loop. A loop can be used to iterate over the rows in Table1 and insert multiple rows into Table2 for each row.

Here's an example query that uses a loop to insert multiple rows from a single row in Table1 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;

In this query, a cursor is used to iterate over the rows in Table1. For each row, a while loop is used to insert multiple rows into Table2 until the [Qty] value is reached.

Conclusion

Introduction

In our previous article, we explored three different ways to insert multiple rows from a single row in SQL Server. In this article, we will answer some frequently asked questions (FAQs) related to this topic.

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

A: The best method depends on the specific requirements of the task. If you need to insert a large number of rows, a recursive Common Table Expression (CTE) or a numbers table may be more efficient. However, if you need to insert a small number of rows, a loop may be sufficient.

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

A: To use a recursive CTE, you need to define a CTE with a recursive clause that selects the first row from the source table and then recursively selects the next row until the desired number of rows is reached. Here's an example 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, 1
FROM RecursiveCTE;

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

A: To use a numbers table, you need to create a table that contains a sequence of numbers and then use a while loop to insert the desired number of rows into the target table. Here's an example query:

DECLARE @Qty INT = 1;

SELECT ID, Name, @Qty AS Qty
INTO #Numbers
FROM (VALUES (1)) AS T (ID);

WHILE @Qty < (SELECT MAX(Qty) FROM Table1)
BEGIN
  SET @Qty = @Qty + 1;
  INSERT INTO #Numbers (ID, Name, Qty)
  SELECT ID, Name, @Qty
  FROM Table1;
END;

INSERT INTO Table2 (ID, Name, Qty)
SELECT ID, Name, Qty
FROM #Numbers;

DROP TABLE #Numbers;

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

A: To use a loop, you need to declare a cursor that iterates over the rows in the source table and then use a while loop to insert the desired number of rows into the target table. Here's an example query:

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;

Q: What are the advantages and disadvantages of using a recursive Common Table Expression (CTE) to insert multiple rows from a single row in SQL Server?

A: The advantages of using a recursive CTE include:

Efficient use of resources
Easy to implement
Can handle large datasets

The disadvantages of using a recursive CTE include:

Can be difficult to understand and debug
May not be supported by all SQL Server versions
Can cause performance issues if not implemented correctly

Q: What are the advantages and disadvantages of using a numbers table to insert multiple rows from a single row in SQL Server?

A: The advantages of using a numbers table include:

Efficient use of resources
Easy to implement
Can handle large datasets

The disadvantages of using a numbers table include:

Requires additional storage space
May not be supported by all SQL Server versions
Can cause performance issues if not implemented correctly

Q: What are the advantages and disadvantages of using a loop to insert multiple rows from a single row in SQL Server?

A: The advantages of using a loop include:

Easy to implement
Can handle small to medium-sized datasets
Can be used with older SQL Server versions

The disadvantages of using a loop include:

Inefficient use of resources
Can cause performance issues if not implemented correctly
May not be supported by all SQL Server versions

Conclusion

In this article, we have answered some frequently asked questions related to inserting multiple rows from a single row in SQL Server. We have discussed the advantages and disadvantages of using a recursive Common Table Expression (CTE), a numbers table, and a loop to achieve this. Each method has its own strengths and weaknesses, and the choice of method depends on the specific requirements of the task.