What is the reason why the results are inaccurate when comparing float fields using where query in Laravel? How to solve this problem?

Comparing floating-point field accuracy problems and solutions using where query in Laravel

When comparing floating point fields using the where clause in Laravel, you sometimes encounter inaccurate results. For example, ->where('odd', '>', 0.3) expects to filter records with odd fields greater than 0.3, but the actual result may contain records with less than 0.3. But using ->whereRaw('odd > 0.3') can get the correct result. This is because there is a precision loss in comparison of floating point numbers inside the where method.

The fundamental reason lies in the accuracy limitations caused by the storage method of floating-point numbers. The where method compares 0.3 as a floating point number, while the floating point representation of the odd field in the database may be slightly different from the memory representation of 0.3 , resulting in inaccurate comparison results. The whereRaw method directly passes the SQL query statement to the database, avoiding Laravel's data type conversion and precision loss.

To avoid this problem and to avoid using whereRaw (as it may pose a risk of SQL injection), the following solutions are recommended:

1. Use string comparison:

Convert floating-point values ​​to strings for comparison. You can bypass Laravel's floating-point conversion and directly compare strings to avoid accuracy loss:

 ->where('odd', '>', (string) 0.3)

2. Use whereBetween and set the accuracy range:

Due to floating point accuracy limitations, a small accuracy range can be set for comparison:

 ->whereBetween('odd', [0.3, 0.30001]) // Adjust the range according to actual accuracy requirements

This filters out records with odd values ​​between 0.3 and 0.30001, effectively solving the accuracy problem. The accuracy range needs to be adjusted according to the actual application scenario.

3. Use database functions for comparison (for example: ROUND ):

If the database supports it, you can use the database's ROUND function to round the floating point numbers and then compare them to reduce the accuracy error:

 ->where(DB::raw('ROUND(odd, 2)'), '>', 0.3) // Keep two decimal places

Which method to choose depends on your specific needs and database system. The string comparison method is simple and direct, but the readability may be slightly worse; the whereBetween method is clearer, but the accuracy range needs to be adjusted carefully; the database function method is more flexible, but it requires understanding of the function support of the database. It is recommended to prioritize the use of string comparison or whereBetween methods to improve the readability and security of the code.

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