SQL EXISTS Function

Introduction to EXISTS in SQL

The EXISTS function is a powerful tool in SQL used for checking the presence of records in a table or subquery. In simple terms, when you want to know whether a record or a set of records exists in a table or subtable, EXISTS can be used. The main difference between EXISTS and other commands lies in its focus on verifying the existence of data rather than the quantity of data.

This function specifically assists the user in confirming whether a related record exists in another table or not, which is usually the case. This might occur when there is a relationship with the corresponding table, and we want to filter data based on the existence or non-existence in another table.

One of the notable advantages of EXISTS is that it can perform better compared to functions like COUNT(), because in EXISTS only checks for existence is enough and there's no need to count all records. Therefore, it can provide better performance in situations where the functionality is more significant than the actual count of records.

Key point in using EXISTS is that it can execute a type of subquery that retrieves the exact amount of existing records in the case of presence of records in the subquery, precisely returning the correct amount.

Currently in real-world applications, EXISTS is often used for validation or prevention of iterative or non-deterministic operations where the data is from a non-accessible source, it is utilized. In this case, it is always crucial to consider how and when this tool is used.

SQL Code Example

    SELECT column_name(s)
    FROM table1
    WHERE EXISTS (
        SELECT column_name
        FROM table2
        WHERE table1.column_name = table2.column_name
    );
    

Line-by-line Explanation of the SQL Code