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Tim Bachmann
Tim Bachmann

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How to write optional filters in SQL

The problem

Let's say you have a rest API with the following endpoint that returns all of the books in your database:

GET /book/
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Your SQL query might look like something like this

SELECT * 
FROM books
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Sometimes you want to only list books, for example, from a specific author. How do we do this in SQL?

Naive solution: String concatenation ✂

One way would be to concatenate your sql query something like this:

const arguments = [];
const queryString = 'SELECT * FROM books WHERE true';
if ( authorFilter != null) {
  queryString  += 'AND author = ?';
  arguments.push(authorFilter);
}
db.query(queryString, arguments); 
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I'm not much of a fan of manually concatenating strings.

The coalesce function 🌟

Most Databases have the function coalesce which accepts a variable amount of arguments and returns the first argument that is not null.

-- Examle
SELECT coalesce(null, null, 'dev.to', null, '@TiimB') as example;

-- Will return 

example
---------
dev.to
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But how will this function help us?

Optional filters with the coalesce function

SELECT * 
FROM books
WHERE 
  author = coalesce(?, author);
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If the filter value is null the coalesce expression will resolve to author
and the comparison author = author will be true.

If on the other hand the value is set for example to Shakespeare then the author will be compared to Shakespeare.

I came across this way to implement optional filters only recently. If you have a more idiomatic way to do this let me know please ✨

If you liked this post please follow me on here or on Twitter under @TiimB 😎

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