Understanding the NOT NULL Constraint and Default Values in SQL Insertions

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Understanding the NOT NULL Constraint and Default Values in SQL Insertions

The Problem Explained

Let's start by examining a scenario where we have a table named currencies. In this table, the prepend_symbol_flg column is defined as NOT NULL and has a default value of false. This means that if you want to create a new entry in the table without explicitly specifying a value for prepend_symbol_flg, the expectation is that the default value should fill in.

The Table Schema

The relevant schema for our currencies table looks like this:

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The INSERT Statement

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When we send a POST request without including prepend_symbol_flg, we'll receive the error message:

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This error arises because the database interprets the absence of prepend_symbol_flg as a prompt to insert NULL, which violates the NOT NULL constraint.

Why the Default Value Doesn't Apply

The misunderstanding here is about how PostgreSQL applies default values:

Default Values: PostgreSQL uses default values for columns only when those columns are not mentioned at all in the INSERT statement.

NULL Values: If you attempt to insert a NULL value—whether intentionally or inadvertently (like when not passing a required field)—this leads to a violation of the NOT NULL constraint.

Practical Illustration

When you execute the INSERT statement without the prepend_symbol_flg, it is assumed you want to set that field to NULL, leading to a violation error rather than using the default false as expected.

To address this problem, you'll need to ensure that your application logic handles defaults correctly before executing the SQL statement.

Option 1: Use Default Value Logic in JavaScript

You can set a default value in your JavaScript code before making the database call, like so:

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Option 2: Create Dynamic Queries

Another approach is to dynamically construct your SQL statement based on which fields are provided in the request. This requires additional checks but ensures that you're only inserting the provided values.

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Conclusion

By following these best practices, you'll avoid common pitfalls and ensure that your applications are withstanding against typical database errors. Remember, every mistake is a learning opportunity in software development!