Understanding SQL Server Intersect – A Guide for Devs

Hello Dev, as you delve deeper into the world of SQL Server, you may have come across the term ‘intersect’. Understanding what this term means and how it works can greatly enhance your SQL Server skills. In this article, we’ll take a detailed look at SQL Server intersect and how it can be used in your queries.

What is SQL Server Intersect?

SQL Server Intersect is a set operator used in SQL queries to return only the shared rows between two or more tables. In other words, it only returns the data that appears in both tables being compared. It’s similar to the inner join operation, but with a few key differences.

One important thing to note is that all tables compared must have the same number of columns, and those columns must have compatible data types. Let’s take a closer look at how SQL Server Intersect works.

The Syntax of SQL Server Intersect

The syntax of SQL Server Intersect is as follows:

SELECT column1, column2, …, columnN
FROM table1
INTERSECT
SELECT column1, column2, …, columnN
FROM table2;

As you can see, it requires the use of two SELECT statements that are separated by the ‘INTERSECT’ keyword. The columns being compared must have the same names and data types in both tables.

Using SQL Server Intersect in Your Queries

Now that you have a basic understanding of the syntax of SQL Server Intersect, let’s explore how it can be used in your queries. The following example demonstrates how to use Intersect to find only the records that exist in both tables:

Example:

Table1		Table2
ID	Name	ID
1	John	1
2	Jane	3
3	Bob	4
4	Mary	5

The following SQL query demonstrates how to use Intersect to find the shared records between Table1 and Table2:

Query:

SELECT ID, Name FROM Table1
INTERSECT
SELECT ID, Name FROM Table2;

Output:

ID	Name
1	John

As you can see from the output, only the record with ID 1 and name John is returned, as it is the only record that appears in both Table1 and Table2.

Key Differences between SQL Server Intersect and Inner Join

While both Intersect and Inner Join return only shared records, they differ in the way they operate. The following are some key differences between the two:

1. Column Order

With Inner Join, the order of the columns in the joined tables doesn’t matter. However, with Intersect, the columns must be in the same order, and have the same data types, in both tables being compared.

2. Null Values

Inner Join will return all the shared records, even if there are null values in some of the columns. Intersect, on the other hand, only returns the records that have non-null values in both tables.

3. Performance

Intersect can be less performant than Inner Join, especially when dealing with large datasets. This is because it requires SQL Server to compare every single row in both tables being compared, which can be time-consuming.

FAQs about SQL Server Intersect

1. Can SQL Server Intersect be used with more than two tables?

Yes, SQL Server Intersect can be used with any number of tables. Simply add additional SELECT statements separated by the ‘INTERSECT’ keyword.

2. How is SQL Server Intersect different from Union?

SQL Server Union and Intersect are both set operators used to combine or compare tables. However, Union returns all the unique records from all tables being compared, while Intersect only returns the shared records between them.

3. Can SQL Server Intersect be used with text or binary data types?

No, SQL Server Intersect can only be used with compatible data types. Text and binary data types are not compatible with Intersect.

4. Can SQL Server Intersect be used to compare columns with different names?

No, the columns being compared must have the same names in both tables being compared. However, you can use aliases to rename columns in your SELECT statements to match the column names in the other table.

5. Is Intersect a standard SQL operator?

Yes, SQL Server Intersect is a standard SQL operator, and is also supported by other SQL database management systems.

Conclusion

As we’ve seen, SQL Server Intersect is a powerful tool that can be used to compare tables and return only the shared records. While it has some key differences from Inner Join, it can be a useful addition to your SQL Server toolkit. By understanding the syntax and proper usage of Intersect, you can write more efficient and effective queries that will help you get the most out of your SQL Server experience.