Скачать или смотреть How to Select All Rows from Two Tables with the Same ID in SQL

Discover how to efficiently retrieve records from two tables using SQL with the same user ID, enhancing your database querying methods.
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How to Select All Rows from Two Tables with the Same ID in SQL

When working with databases, it is common to encounter scenarios where you need to retrieve data from multiple tables based on a shared identifier, such as user_id. This article will guide you through the process of selecting all relevant rows from two tables, ensuring a clear understanding of SQL queries and joins.

The Problem: Retrieving Rows from Two Tables

You may find yourself in a situation where you have two tables containing related data. For example, let’s consider the following structure:

Tables Overview

TABLE1

user_id

value1

value2

TABLE2

user_id

value3

value4

Your task is to retrieve all lines from both tables where the user_id is 1. You have attempted a query using an INNER JOIN, but this resulted in a single row that combined the values from both tables into one output, which is not what you needed.

The Solution: Using UNION ALL

To achieve the desired result of retrieving all rows separately, you can use the UNION ALL SQL command. This command allows you to merge the results of two SELECT statements while displaying each table's rows distinctly.

Step-by-Step Query Breakdown

Basic Structure: The overall strategy is to select rows from each table and combine them using UNION ALL.

Constructing the SQL Query:
Here's how you can structure the query:

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

The first part selects records from TABLE1 and labels them with 't1'.

The second part selects from TABLE2 and labels those rows with 't2'.

UNION ALL combines these results into a single dataset, which preserves the original rows from both tables.

Important Details to Consider

Data Type Compatibility: It's essential that the selected columns have matching data types across both tables. For instance, value1 and value3 must be compatible, as well as value2 and value4.

Handling Different Data Types: If the data types differ, you can modify the query to return NULLs for the missing values:

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

This approach allows you to still keep the structure intact while indicating which table the values originated from.

Conclusion

By effectively utilizing the UNION ALL command, you can easily retrieve and structure data from multiple tables based on a common identifier. This not only enhances your querying capabilities but also improves the readability and usability of your results. With this method, future database tasks will become more straightforward, allowing for efficient data manipulation and retrieval in SQL.

Feel free to experiment with these SQL queries in your database setup, and see how they can help streamline your data access solutions!