Understanding the Order of CAST() and COALESCE() in MariaDB

MariaDB is a popular open-source relational database management system known for its high performance and reliability. One of the key features of MariaDB is its ability to handle JSON data, which has become increasingly important in modern applications. However, when working with JSON data, it’s essential to understand how various functions interact with each other.

In this article, we’ll explore the order of operations between CAST() and COALESCE() in MariaDB, which can sometimes lead to unexpected results. We’ll delve into the underlying mechanics of these functions and provide practical examples to illustrate their behavior.

What are JSON_EXTRACT() and JSON_VALUE()? A Primer

Before diving into the intricacies of CAST() and COALESCE(), let’s take a brief look at two fundamental JSON functions in MariaDB: JSON_EXTRACT() and JSON_VALUE().

JSON_EXTRACT()

JSON_EXTRACT() returns a JSON object found at a specific path within a JSON document. It takes the following syntax:

JSON_EXTRACT(json, path)

The path parameter specifies the location of the desired value within the JSON document. For example:

SELECT JSON_EXTRACT('{"name": "John", "age": 30}', '$.name');
-- Returns: "John"

In this case, JSON_EXTRACT() returns a string literal "John", which is the value found at the path $.name within the JSON document.

JSON_VALUE()

On the other hand, JSON_VALUE() returns a scalar value from a specific path within a JSON document. It takes the following syntax:

JSON_VALUE(json, path)

The path parameter specifies the location of the desired value within the JSON document. For example:

SELECT JSON_VALUE('{"name": "John", "age": 30}', '$.name');
-- Returns: 30

In this case, JSON_VALUE() returns an integer value 30, which is the value found at the path $.name within the JSON document.

The Problem with CAST() and COALESCE()

Now that we’ve covered the basics of JSON_EXTRACT() and JSON_VALUE(), let’s examine the original question and understand why using these functions in the wrong order can lead to unexpected results.

The query in question uses both CAST() and COALESCE():

CAST(COALESCE(JSON_EXTRACT(item.price_details, "$.shipping.price"), 0) AS FLOAT) AS shippricecoalfloat

COALESCE(CAST(JSON_EXTRACT(item.price_details, "$.shipping.price") AS FLOAT), 0) AS shippricefloatcoal

Here’s what happens when we evaluate these expressions:

1. JSON_EXTRACT(item.price_details, "$.shipping.price"): This function returns a JSON object containing the value of the shipping price.
2. CAST(... AS FLOAT): This function converts the result from step 1 to a floating-point number. However, if the original value is stored as a string with double quotes (e.g., "10"), this conversion will not work as expected.
3. COALESCE(..., 0): This function returns the first non-NULL value it encounters.

The problem arises when we use JSON_EXTRACT() before casting to FLOAT. As mentioned earlier, JSON_EXTRACT() returns a JSON object, which may contain string values with double quotes. When we try to cast these strings to floats using CAST(), MariaDB will convert them to strings instead of numbers.

For example:

SELECT CAST(JSON_EXTRACT('{"shipping.price": "10"}', '$.shipping.price') AS FLOAT);
-- Returns: "10"

In this case, the result is still a string with double quotes, which means that subsequent operations involving this value will not work as expected.

The Solution

To fix this issue, we need to use JSON_VALUE() instead of JSON_EXTRACT(). Here’s the corrected query:

SELECT
    item_id,
    JSON_VALUE(price_details, "$.shipping.price") AS shipprice,
    CAST(COALESCE(JSON_VALUE(price_details, "$.shipping.price"), '0') AS FLOAT) AS shippricecoalfloat,
    COALESCE(CAST(JSON_VALUE(price_details, "$.shipping.price") AS FLOAT), 0) AS shippricefloatcoal
FROM `item`
WHERE order_id = 109517;

As we can see, using JSON_VALUE() ensures that the resulting value is a floating-point number, which can then be converted and compared using COALESCE().

Additional Considerations

There’s another important aspect to consider when working with JSON data in MariaDB: data type conversion. When storing numeric values as strings with double quotes (e.g., "10"), this can lead to unexpected results.

To avoid these issues, it’s essential to ensure that your JSON data is stored correctly and that you use the right functions for your specific use case. As a general rule of thumb:

• Use JSON_VALUE() when retrieving scalar values from a JSON document.
• Avoid using JSON_EXTRACT() when working with floating-point numbers or decimal values.
• Always verify that your JSON data is stored correctly, including numeric values without double quotes.

Conclusion

In this article, we explored the order of operations between CAST() and COALESCE() in MariaDB, which can sometimes lead to unexpected results. By understanding how these functions interact with each other and using the correct syntax for working with JSON data, you can avoid common pitfalls and ensure that your queries produce accurate and reliable results.

Remember to always verify your assumptions and test your queries thoroughly to ensure that they work as expected in different scenarios.

Last modified on 2024-06-04