Manual:$wgShowSQLErrors

When set to  (default) any database error message will also say "(SQL query hidden)" rather than showing the SQL which caused the error. You will then only see "A database query error has occurred. This may indicate a bug in the software.". Set $wgShowSQLErrors to  to see the actual query, which caused the error.

Details
Clearly showing the SQL will aid in debugging in the event of a database error, however...
 * Showing SQL to all users, will give crackers some clues about the database structure of your website, and might help them find security weaknesses.
 * SQL database errors are not supposed to occur during any normal MediaWiki operation, and so with luck you will never need to improve the error information
 * If you receive the error  from the API, enabling this setting will display a more descriptive error message in the info field than the default.

If you do see an error including "(SQL query hidden)", it can be useful to set this to $wgShowSQLErrors = true; temporarily.

This variable replaces $wgIgnoreSQLErrors, which should no longer be used.