Efficiently Insert a Tag in PostgreSQL Only If It Doesn't Already Exist

Learn how to insert a tag in PostgreSQL only if it does not already exist and return the ID with an efficient SQL query.
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Efficiently Insert a Tag in PostgreSQL Only If It Doesn't Already Exist
When working with databases, particularly PostgreSQL, there are often times when you need to insert a new record only if it doesn't already exist. This is a common scenario when managing tags or any similar set of unique data entries. In this guide, we'll explore how to efficiently achieve this using SQL in PostgreSQL.
The Challenge
Imagine you're working with a table named tags, and it has two columns: id and name. The primary goal is to insert a new tag into this table only if a tag with the same name does not already exist. Additionally, if the tag does exist, you need to retrieve its ID.
Solution
PostgreSQL offers several ways to handle this scenario. The most efficient and common method involves using the INSERT ... ON CONFLICT DO NOTHING statement combined with the RETURNING clause.
Here’s how you can do it:
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Breakdown of the SQL Query
INSERT INTO tags (name) VALUES ('your_tag_name'): This part attempts to insert a new tag with the name 'your_tag_name'.
ON CONFLICT (name) DO NOTHING: This clause tells PostgreSQL to do nothing if there is a conflict on the name column (i.e., if a tag with the same name already exists).
RETURNING id: After inserting a new tag or doing nothing (in case of a conflict), PostgreSQL will return the id of the tag.
Handling the Result in Application Code
If no new row is inserted (because a tag with the same name already exists), the RETURNING clause might not return any rows. To ensure you always get the id, even when no new row is inserted, you can use a common table expression (CTE) to handle this:
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Explanation of the Enhanced SQL Query
WITH ins AS (... RETURNING id): This CTE attempts to insert the new tag and returns its id if the insert is successful.
SELECT id FROM ins UNION ALL SELECT id FROM tags WHERE name = 'your_tag_name': If the insert isn't successful (due to conflict), this part queries the tags table to get the existing tag's id.
LIMIT 1: Ensures that only one id is returned, regardless of the outcome.
Using this approach, you ensure that you always get the id of the tag, whether it was inserted or already existed.
Conclusion
Inserting records conditionally in PostgreSQL can be efficiently achieved using the INSERT ... ON CONFLICT DO NOTHING statement combined with the RETURNING clause or a common table expression. This method ensures that you can insert a new tag if it doesn’t exist or return the id of an existing tag seamlessly. This approach is not only efficient but also simplifies your database operations by reducing the need for complex application-side logic.