MERGE Statement in SQL

Last Updated: 22nd June, 2023

Overview

This lesson briefly introduces the SQL command called "MERGE". This command allows you to perform a combination of different operations, such as insert, update, and delete, all in one statement. While the statement itself may be brief, there are several practical applications that can be explored in greater detail.

What is Merge Statement?

MERGE is a SQL command that combines records from two or more tables. This is commonly used when a table needs to be updated with data from another table. For example, an industry might use the MERGE command to update customer records with new information from a separate table. The customer table might have basic information like name, address, and phone number. The separate table might have additional information like age, gender, and income level. The MERGE command would then be used to combine the two tables and update the customer records with the additional information. Let's look into it closely.

In SQL, the MERGE statement is a powerful tool that combines multiple operations into a single statement. It is also known as the UPSERT operation, which means it updates if the record exists and inserts if the record does not exist.

The MERGE statement combines INSERT, UPDATE, and DELETE operations into a single statement. It is used to perform operations on a target table based on the data from a source table. The source table is typically a temporary or derived table containing data you want to merge into the target table.

The basic syntax for a MERGE statement is as follows:

MERGE INTO target_table USING source_table 
ON join_condition 
WHEN MATCHED THEN 
    UPDATE SET column1 = value1 [, column2 = value2 ...] 
WHEN NOT MATCHED THEN    
    INSERT (column1 [, column2 ...]) VALUES (value1 [, value2 ...])
 

Let's break down the components of the MERGE statement:

• MERGE INTO: This specifies the target table that you want to update.
• USING: This specifies the source table that you want to merge into the target table.
• ON: This specifies the join condition that determines how the rows in the source table match up with the rows in the target table.
• WHEN MATCHED: This specifies what to do when a matching row is found in both the source and target tables. You can update the target table with data from the source table, delete the row from the target table, or do nothing.
• WHEN NOT MATCHED: This specifies what to do when a row in the source table does not have a matching row in the target table. You can insert the row into the target table or do nothing.

Let's look at an example to better understand how the MERGE statement works. Suppose we have a target table called customers that contains the following data:

We also have a source table called new_customers that contains the following data:

To merge the data from the new_customers table into the customer's table, we can use the following MERGE statement:

 
MERGE INTO customers c USING new_customers nc 
ON c.id = nc.id 
WHEN MATCHED THEN 
    UPDATE SET c.name = nc.name, c.email = nc.email 
WHEN NOT MATCHED THEN 
    INSERT (id, name, email) VALUES (nc.id, nc.name, nc.email);

This statement will update the row in the customer's table with id 1 because it matches the corresponding row in the new_customers table. It will also insert the row from the new_customers table with id 3 into the customer's table because there is no matching row in the target table.

After the MERGE operation, the customer's table will look like this:

Use Cases for Merge Statements in SQL

The merge statement in SQL is useful in a wide range of scenarios. Some common use cases include:

• Updating data in a target table based on data from a source table.
• Inserting new data into a target table based on data from a source table.
• Synchronizing data between two tables or databases.
• Merging data from different tables or databases into a single table.

Best Practices for Using Merge Statements in SQL

When utilizing the merge statement in SQL, it's imperative to take after a few best practices to guarantee that your code is productive, precise, and simple to preserve. A few best hones include:

• Always specify the columns that you need to insert or upgrade within the target table.
• Utilize a brief table or a CTE (common table expression) to preprocess the information sometime recently merging it into the target table.
• Utilize proper indexing to speed up the combined operation, particularly for huge tables.
• Test your code altogether to guarantee that it produces the anticipated results and doesn't present any data errors.

Conclusion

The industry used the MERGE command in SQL to combine two separate tables. The first table contained basic information like name, address, and phone number, while the second table contained additional information like age, gender, and income level. The customer records were successfully updated with the additional information using the MERGE command.

Key Takeaways

1. The merge statement merges data from one or more tables into a target table based on a condition.
2. The merge statement syntax includes the target table, source table, condition, and clauses for updating and inserting data.
3. The merge statement is useful in scenarios such as updating data in a target table, inserting new data, synchronizing data, and merging data from different tables.
4. Best practices for using the merge statement include specifying columns to insert or update, preprocessing data with a temporary table or CTE, using proper indexing, and testing code thoroughly.

Quiz

1. What is the purpose of the MERGE statement in SQL? 

1. To select data from multiple tables 
2. To insert data into multiple tables 
3. To update, insert, and delete data in a single statement 
4. To create a new table from existing tables

Answer: c. To update, insert, and delete data in a single statement.

2. What does the MERGE INTO clause specify? 

1. The columns to be updated in the target table 
2. The source table to merge data from 
3. The target table to update 
4. The join condition for the tables

Answer: c. The target table to update.

3. When is the WHEN MATCHED clause used in a MERGE statement? 

1. When a matching row is found in both the source and target tables 
2. When a row is found in the source table that does not have a matching row in the target table 
3.  When a row is found in the target table that does not have a matching row in the source table 
4. When all rows in the source table need to be inserted into the target table

Answer: a. When a matching row is found in both the source and target tables.

4. What is a common use case for the MERGE statement in SQL? 

1. Creating a new table from existing tables 
2.  Deleting data from a table 
3. Updating data in a target table based on data from a source table 
4. Sorting data in a table

Answer: c. Updating data in a target table based on data from a source table.

5. What is the best practice for using the MERGE statement in SQL? 

1. Avoid indexing to speed up the merge operation 
2. Don't specify the columns to be inserted or updated in the target table 
3. Test your code thoroughly before executing it on the database 
4. Use a subquery instead of a temporary table or CTE

Answer: c. Test your code thoroughly before executing it on the database.