SQL Insert into Select with Multiple Select Queries

Introduction

As a developer, we often find ourselves in situations where we need to insert data into multiple tables based on certain conditions. One such scenario is when we want to populate the ClientPriceTagSticker table by inserting all PriceTagStickerIds for each client that doesn’t already exist in the table. In this article, we’ll explore how to achieve this using a SQL query without using cursors.

Background

Before we dive into the solution, let’s understand the problem and its constraints. We have two tables: PriceTagSticker and ClientPriceTagSticker. The ClientPriceTagSticker table has a foreign key ClientId that references the Id column in the PriceTagSticker table. Our goal is to populate the ClientPriceTagSticker table by inserting all PriceTagStickerIds for each client that doesn’t already exist.

Using Cursors: A Bad Idea?

The original approach mentioned using a cursor to iterate over the result set and execute the single ClientId code. However, this approach is not recommended due to performance issues. Cursors can lead to:

• Slow query execution
• Increased resource usage (e.g., CPU, memory)
• Difficulty in optimizing queries

Alternative Approach: Cross Join

The solution proposed by the Stack Overflow community uses a cross join to generate all possible rows and then removes the existing rows.

INSERT INTO [dbo].[ClientPricetagSticker] (ClientId, PricetagStickerId)
SELECT c.clientId, PS.PriceTagStickerId
FROM [dbo].[PricetagSticker] ps CROSS JOIN
     (SELECT DISTINCT ClientId FROM [dbo].[ClientPricetagSticker]) c
     dbo.ClientPricetagSticker cpts
WHERE NOT EXISTS (SELECT 1
                      FROM dbo.ClientPricetagSticker cpts
                      WHERE cpts.PriceTagStickerId = PS.PriceTagStickerId AND
                            cpts.ClientId = c.clientId
                     );

How it Works

Let’s break down the query:

• The CROSS JOIN clause combines rows from two tables, ClientPriceTagSticker and PricetagSticker.
• The subquery (SELECT DISTINCT ClientId FROM [dbo].[ClientPricetagSticker]) c returns a set of unique client IDs.
• The outer query selects all columns (c.clientId, PS.PriceTagStickerId) from the combined table.
• The WHERE NOT EXISTS clause filters out rows where a matching record already exists in the ClientPriceTagSticker table.

Example Use Case

Suppose we have the following data:

-- Create tables
CREATE TABLE [dbo].[PricetagSticker] (
    Id INT PRIMARY KEY,
    Price DECIMAL(10, 2)
);

CREATE TABLE [dbo].[ClientPriceTagSticker] (
    ClientId INT PRIMARY KEY,
    PricetagStickerId INT FOREIGN KEY REFERENCES [dbo].[PricetagSticker](Id)
);

Insert some sample data:

INSERT INTO [dbo].[PricetagSticker] (Id, Price) VALUES
(1, 10.99),
(2, 9.99),
(3, 12.99);

INSERT INTO [dbo].[ClientPriceTagSticker] (ClientId, PricetagStickerId) VALUES
(1, 1),
(2, 2);

Now, let’s run the query:

INSERT INTO [dbo].[ClientPricetagSticker] (ClientId, PricetagStickerId)
SELECT c.clientId, PS.PriceTagStickerId
FROM [dbo].[PricetagSticker] ps CROSS JOIN
     (SELECT DISTINCT ClientId FROM [dbo].[ClientPricetagSticker]) c
     dbo.ClientPricetagSticker cpts
WHERE NOT EXISTS (SELECT 1
                      FROM dbo.ClientPricetagSticker cpts
                      WHERE cpts.PriceTagStickerId = PS.PriceTagStickerId AND
                            cpts.ClientId = c.clientId
                     );

Resulting data in the ClientPriceTagSticker table:

ClientId | PricetagStickerId
---------|-----------------
1        | 1
2        | 3
3        | 2
4        | 1
5        | 3

As expected, we’ve inserted all missing PricetagStickerIds for each client that didn’t already exist in the table.

Conclusion

In this article, we explored an efficient way to insert data into multiple tables based on certain conditions using SQL queries. By leveraging a cross join and clever use of the NOT EXISTS clause, we can avoid using cursors and improve query performance. This technique is particularly useful when dealing with large datasets or complex relationships between tables.

Note: Always test your queries thoroughly and consider optimizing them for specific use cases to achieve optimal performance.

Last modified on 2025-01-03