Comparison with trailing spaces in MySQL

It's all stated there in the documentation. I've quoted the important points here. But I would suggest to go through the full documentation

VARCHAR values are not padded when they are stored. Trailing spaces are retained when values are stored and retrieved, in conformance with standard SQL.

On the other hand, CHAR values are padded when they are stored but trailing spaces are ignored when retrieved.

All MySQL collations are of type PADSPACE. This means that all CHAR, VARCHAR, and TEXT values in MySQL are compared without regard to any trailing spaces. “Comparison” in this context does not include the LIKE pattern-matching operator, for which trailing spaces are significant.

Explanation: Trailing spaces are ignored while comparing strings using comparison operator ('='). But trailing spaces are significant for LIKE (pattern matching operator)

If your column is from type CHAR and not VARCHAR, than this is correct. On CHAR-Fields will trailing blanks on comparing ignored! So

field = ''
field = '    '

are the same.

This is documented behaviour.

The MySQL documentation for LIKE mentions

trailing spaces are significant, which is not true for CHAR or VARCHAR comparisons performed with the = operator:

SQL Server works the same way.