As a SQL Server developer or database administrator, you may come across situations where your query returns more data than you expect. This can be confusing and may lead to incorrect results if not understood properly. In this article, we will discuss the concept of Inner Join in SQL Server and how it can sometimes return more records than exist in the table.
Let’s start with a simple example. Imagine you have two tables, FirstTable and SecondTable, both containing three rows with values 1, 2, and 3. If you perform an Inner Join between these two tables using the equal to (=) sign, the result will be the same as the number of rows or fewer than the number of rows in the table. This is because the data is unique in both tables.
However, if you change the condition to not equal to (<>) sign, the result set will contain more records than the number of rows in the table. The not equal to condition returns the result that was excluded in the query where the equal to condition was used. This can be a bit counterintuitive, but it’s important to understand that Inner Join returns results based on the condition specified in the JOIN condition.
Additionally, if you have a table where the data is not unique, Inner Join can return more records than one of the tables. This happens when there are multiple rows in the other table that satisfy the join condition. In such cases, Inner Join will return all the matching rows, resulting in more records than exist in the table.
Let’s take a look at the following example:
CREATE TABLE FirstTable ( Col1 INT )
CREATE TABLE SecondTable ( Col1 INT )
INSERT INTO FirstTable ( Col1 ) VALUES ( 1 ), ( 2 ), ( 3 )
INSERT INTO SecondTable ( Col1 ) VALUES ( 2 ), ( 2 ), ( 2 )
If we perform an Inner Join between FirstTable and SecondTable using the equal to (=) sign, the result will be:
SELECT f.Col1
FROM FirstTable f
INNER JOIN SecondTable s ON s.Col1 = f.Col1
The result will be:
Col1
----
2
2
2
Now, if we change the condition to not equal to (<>) sign, the result will be:
SELECT f.Col1
FROM FirstTable f
INNER JOIN SecondTable s ON s.Col1 <> f.Col1
The result will be:
Col1
----
1
3
It’s important to note that if you combine the results of the equal to (=) and not equal to (<>) conditions, you will get the same result as a CROSS JOIN of both tables.
In summary, Inner Join in SQL Server can return more records than exist in the table. This happens when the join condition allows for multiple matching rows or when the condition is not restrictive enough. It’s crucial to understand the behavior of Inner Join and carefully define your join conditions to ensure accurate results.
Feel free to explore further and experiment with different join conditions to deepen your understanding of Inner Join in SQL Server.
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