Escapes data for use in a MySQL query.
Usually you should prepare queries using wpdb::prepare(). Sometimes, spot-escaping is required or useful. One example is preparing an array for use in an IN clause.
NOTE: Since 4.8.3, '%' characters will be replaced with a placeholder string, this prevents certain SQLi attacks from taking place. This change in behaviour may cause issues for code that expects the return value of esc_sql() to be useable for other purposes.