Understanding SQL: JSON and Cursors

February 3, 2025

Introduction to SQL

Structured Query Language (SQL) is the standard language for managing and manipulating relational databases. It is used to insert, update, delete, and retrieve data efficiently.

JSON in SQL

JSON (JavaScript Object Notation) is a lightweight data-interchange format. Modern databases support JSON to store and query structured data flexibly.

JSON Functions in SQL

Most relational databases, like MySQL, PostgreSQL, and SQL Server, provide functions to work with JSON. Here are some common JSON functions:

MySQL JSON Functions:
- JSON_OBJECT() – Creates a JSON object
- JSON_ARRAY() – Creates a JSON array
- JSON_EXTRACT() – Extracts data from a JSON document
- JSON_UNQUOTE() – Unquotes a JSON value

PostgreSQL JSON Functions:
- to_json() – Converts data to JSON
- jsonb_extract_path_text() – Extracts a specific key’s value
- json_agg() – Aggregates JSON values into an array

SQL Server JSON Functions:
- JSON_VALUE() – Extracts a scalar value from JSON
- JSON_QUERY() – Extracts an array or object
- OPENJSON() – Parses JSON into rows and columns

Example: Using JSON in MySQL

SELECT JSON_OBJECT('name', 'Alice', 'age', 25) AS user_data;

This returns: {"name": "Alice", "age": 25}

SQL Cursors

A cursor in SQL is used to process a result set row-by-row. It is beneficial when working with large data sets where processing each row individually is required.

Cursor Operations:

Declare the Cursor – Define the cursor to fetch data from a specific table.
Open the Cursor – Prepare it for fetching rows.
Fetch Data – Retrieve row-by-row data.

Close the Cursor – Release the resources.

Example: Using a Cursor in MySQL:

 DECLARE done INT DEFAULT 0;
DECLARE user_name VARCHAR(100);
DECLARE cur CURSOR FOR SELECT name FROM users;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = 1;

OPEN cur;

LOOP_LABEL: LOOP
    FETCH cur INTO user_name;
    IF done THEN
        LEAVE LOOP_LABEL;
    END IF;
    
    -- Process the fetched row
    SELECT user_name;
END LOOP;

CLOSE cur;

When to Use Cursors

When row-by-row processing is necessary.
When working with complex business logic.
When handling large data sets that cannot be processed at once.

Drawbacks of Cursors

Slower compared to set-based operations.
Consumes more memory and resources.
Should be avoided when alternative solutions (e.g., joins, subqueries) exist.

Conclusion

JSON and Cursors in SQL provide powerful functionalities for handling structured and semi-structured data. JSON enhances flexibility, while Cursors allow row-wise processing. However, efficient usage is necessary to optimize performance.

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