CTE, subquery or view?

Hello, dear friends! Today we will look at the various approaches that developers use to work with data in the database. In today's development world, where information is becoming more and more abundant and speed of data retrieval is of great importance, the ability to efficiently extract and process data is becoming an integral part of the work of many SQL professionals (especially those who work with high-load systems and DWH). We'll talk about techniques like Common Table Expressions (CTE), subqueries, views, and materialized views.

CTE (Common Table Expression) and their uses

Definition: A CTE (Common Table Expression) is a temporary result set that is defined within the execution of a single statement (SELECT, INSERT, UPDATE, or DELETE). CTEs allow you to simplify complex queries, making them more readable and maintainable.

Characteristics of CTE

CTEs can be materialized in memory during query execution, avoiding re-execution of the same SQL query if it is used multiple times in the main query.

Syntax:

WITH CTE_Name AS (
    SELECT column1, column2
    FROM table_name
    WHERE condition
)
SELECT *
FROM CTE_Name;

Benefits of CTE:

1. Readability: CTEs simplify complex queries by breaking them down into logically coherent parts, making the code easier to understand.

2. Avoiding Multiple Executions: When the same set of data is required in multiple places in a query, using CTE allows the results to be materialized once and used multiple times without re-fetching from the database, resulting in more efficient execution.

3. Recursion: CTEs support recursive queries, which allows you to perform complex hierarchical queries (for example, when working with parent and child records).

4. Modularity: CTEs help encapsulate logic, making it reusable within a single query, and making SQL code easier to maintain and modify.

When to use CTE:

• When working with complex joins or multiple aggregations, where repeating the same subtask can significantly reduce performance.

• In situations where it is necessary to create a recursive query (for example, to retrieve data from a hierarchical structure).

• To increase the readability and structure of queries, especially if they contain several levels of nesting.

Subqueries and their use

Definition: A subquery is a nested query that is placed inside another SQL query. It can return data which can then be used in the main query.

Syntax:

SELECT column1, column2
FROM table_name
WHERE column1 IN (
    SELECT column1
    FROM another_table
    WHERE condition
);

SELECT 
column1,
(
    SELECT column1
    FROM another_table as at
    WHERE at.id = t.another_table_id
) as column1_from_another_table
FROM table_name as t

Benefits of Subqueries:

• Inline queries: Subqueries allow you to use the result directly without creating a temporary structure.

• Flexibility: They can be used in SELECT, INSERT, UPDATE and DELETE statements.

When to use subqueries:

• When the calculation is simple and does not require reuse.

• When you need to filter data based on the results of another query.

Presentation and their use

Definition: A view is a virtual table based on the result of an SQL query. Views do not physically store data, but provide a way to simplify and name complex queries.

Syntax:

CREATE VIEW View_Name AS
SELECT column1, column2
FROM table_name
WHERE condition;

Benefits of Views:

• Data Security: Restrict access to specific rows or columns.

• Reuse: Easy to reuse across multiple queries.

When to Use Views:

• When it is necessary to encapsulate complex queries.

• To increase security by restricting access to data.

• To avoid code duplication

Materialized View and Their Uses

Definition: A materialized view is a database object that contains the results of a query and stores them physically. Unlike regular views, materialized views can be updated periodically and provide better performance for complex aggregation queries.

Syntax:

CREATE MATERIALIZED VIEW Materialized_View_Name AS
SELECT column1, column2
FROM table_name
WHERE condition;

Benefits of Materialized Views:

• Increased productivity: Significantly speeds up queries by pre-calculating complex joins and aggregations.

• Storing Results: Results are stored, allowing faster access than if they had to be calculated each time.

When to Use Materialized Views:

• When the original data rarely changes.

• When performance issues arise due to complex queries or aggregations.

temporary tables and their use

Definition: A temporary table is a short-lived table that is created during a user session. They may temporarily store results and are automatically deleted when the session ends.

Syntax:

CREATE TEMPORARY TABLE Temp_Table_Name (
    column1 datatype,
    column2 datatype
);

Benefits of Temporary Tables:

• Session scope: Data is stored temporarily without affecting the underlying database schema.

• Flexibility of structure: They can be dynamically structured as needed.

• Removal and isolation: Automatically deleted at the end of the session

When to use temporary tables:

• For complex transformations where intermediate steps need to be preserved.

• When you need to temporarily store a large number of rows.

When to use each design?

Conclusion

In conclusion, choosing the right SQL design—be it CTEs, subqueries, views, materialized views, or temporary tables—plays a key role in the efficiency and usability of working with your data. Each of these solutions has its own strengths and weaknesses, and their application depends on the specific requirements of your project.

For example, when you are faced with a situation where two queries use the same calculation based on data from the same tables, creating a view may be the optimal solution to not only improve the readability of the code, but also improve its performance. In cases where it is necessary to select data based on pre-processed results and reuse them within a single query, it is worth paying attention to CTE. This approach helps avoid code duplication and makes the query more readable.

If your task is to store temporary data for further calculations or checking results, temporary tables will be the best choice. Their use allows flexible management of intermediate results without affecting the main database tables. In cases where your data is updated infrequently and you need quick access to complex queries, it's worth considering materialized views, which can significantly speed things up.