Joe Celko's SQL for Smarties, 3rd Edition

12.1. Tricks with Patterns

The ‘_’ character tests much faster than the ‘%’ character. The reason is obvious: the parser that compares a string to the pattern needs only one operation to match an underscore before it can move to the next character, but has to do some look-ahead parsing to resolve a percentage sign. The wildcards can be inserted in the middle or beginning of a pattern. Thus, ‘B%K’ will match ‘BOOK’, ‘BLOCK’, and ‘BK’, but it will not match ‘BLOCKS’.

The parser would scan each letter and classify it as a wildcard match or an exact match. In the case of ‘BLOCKS’, the initial ‘B’ would be an exact match and the parser would continue; ‘L’, ‘O’, and ‘C’ have to be wildcard matches, since they don’t appear in the pattern string; ‘K’ ...