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Learn how to effectively use `INSERT` statements with subqueries in `PostgreSQL` to manage data across multiple tables, ensuring foreign key relations are respected.
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Mastering the PostgreSQL Insert with Subqueries Across Multiple Tables

When working with databases, one key operation is inserting records into tables that reference data from other tables, particularly when using foreign keys. If you're learning SQL with PostgreSQL, you may have encountered a situation where you need to insert a record into a table by gathering data from multiple others. This can be a bit tricky, especially if you're unfamiliar with SELECT subqueries. In this guide, we'll explore how to effectively perform this task using a practical example.

The Problem

Imagine you have three tables: person, city, and employee. Your goal is to insert a new record into the employee table, where each employee is associated with a person from the person table and a city from the city table, using foreign keys. Here's the syntax for creating these tables:

[[See Video to Reveal this Text or Code Snippet]]

The Challenge

The issue arises when attempting to insert a new employee record by fetching data from the person and city tables using subqueries in the following manner:

[[See Video to Reveal this Text or Code Snippet]]

In its current form, this statement is incorrect because it doesn't properly handle multiple SELECT statements within the INSERT. You need a more effective way to gather this data for insertion.

The Solution

To overcome this obstacle, we can leverage SQL JOIN operations, or consider simplifying the relationships among the tables. Let's break the solution down into manageable steps.

1. Understanding Joins

In SQL, a JOIN allows you to combine records from two or more tables based on a related column between them. In our case, however, the simple structure of our person and city tables does not provide common attributes to join on directly. Here's a basic framework for using a join in SQL:

[[See Video to Reveal this Text or Code Snippet]]

2. Defining Relationships

Since it's not practical to join the tables due to their structure, consider altering your design. If we introduce a column to the person table that links to the city, we can create as follows:

[[See Video to Reveal this Text or Code Snippet]]

After establishing this relationship, you can easily insert into the employee table with a simple join:

[[See Video to Reveal this Text or Code Snippet]]

3. Using Cross Join If Necessary

If you don't want to establish a relationship, but still wish to create records in the employee table by linking every person with every city, you can use a CROSS JOIN. This method combines all rows from both tables:

[[See Video to Reveal this Text or Code Snippet]]

Caution: This method can lead to a significant increase in the number of records. For instance, if you have 20 people and 40 cities, you will wind up with 800 records in the employee table, which might not be desirable.

4. Conclusion

To effectively insert records into the employee table with references from both person and city, it's crucial to define relationships distinctly and utilize SQL operations like joins or cross joins appropriately. Remember that the general pattern requires you to craft a SELECT that matches the columns you need to insert, ensuring nothing is overlooked.

With these tools, you'll be better equipped to manage and manipulate your PostgreSQL databases efficiently. Happy querying!