Скачать или смотреть How to Select from Different Tables in PostgreSQL Based on Condition

Discover the best method to run SQL select statements conditionally in PostgreSQL using UNION for streamlined and effective queries.
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Understanding Conditional Selection in PostgreSQL

When working with relational databases like PostgreSQL, you often encounter scenarios where you need to select data based on certain conditions. One common challenge is to fetch data from multiple tables conditionally. For instance, how can you select rows from one table if a specific condition is met, or from another if it's not? In this post, we will explore a structured approach to solve this problem effectively.

The Problem

Suppose you have two tables, table_B and table_C, and a third table, table_A, that contains a flag indicating which table to select from. The goal is to fetch records from table_B if the flag is true and from table_C if the flag is false. However, simply trying to write a case statement to switch between the two tables will lead to subquery issues, particularly if more than one row is returned.

The format of the problematic query looks something like this:

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

Running the above will not work due to the error: "more than one row returned by a subquery used as an expression."

The Solution: Using UNION

To resolve this issue, we can leverage the UNION operator. UNION allows you to combine the results of two or more SELECT statements. Here’s how you can structure the query to meet the requirement:

Step-by-Step Breakdown:

Select from Table_B: You fetch all rows from table_B where the flag in table_A is true.

Select from Table_C: You fetch all rows from table_C where the flag in table_A is false.

Combine Results: Use UNION to combine the results from both selects.

The Final Query

Here’s how the query will look:

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

Explanation of the Query

Single Subquery for Flags: The subqueries check the value of the flag only once for each table. If the flag is true, it fetches results from table_B. If false, it retrieves results from table_C.

UNION ALL Operator: We use UNION ALL instead of UNION because UNION ALL includes duplicate records. If you're certain there will be no duplicates, you can use UNION for uniqueness.

Handling Multiple Rows: This solution assumes that table_A has only one relevant row for the flag. If there are multiple rows and you need to consider different conditions, you can modify the subquery conditions accordingly.

Conclusion

By using the UNION operator, you can efficiently select between two tables based on a condition in PostgreSQL. This approach not only resolves the subquery issue but also ensures that you can scale your query as needed in the future. Whether working on simple or complex queries, understanding how to structure these operations is crucial for effective database management.

Feel free to reach out if you have more questions about crafting the perfect SQL query or need assistance with other PostgreSQL challenges!