DISTINCT | UNIQUESpecify DISTINCT or UNIQUE if you want the database to return only one copy of each set of duplicate rows selected. These two keywords are synonymous. Duplicate rows are those with matching values for each expression in the select list.Restrictions on DISTINCT and UNIQUE Queries These types of queries are subject to the following restrictions:When you specify DISTINCT or UNIQUE, the total number of bytes in all select list expressions is limited to the size of a data block minus some overhead. This size is specified by the initialization parameter DB_BLOCK_SIZE.这是官方文档的解释；看红字部分；
These types of queries are subject to the following restrictions:When you specify DISTINCT or UNIQUE, the total number of bytes in all select list expressions is limited to the size of a data block minus some overhead. This size is specified by the initialization parameter DB_BLOCK_SIZE.这是官方文档的解释；看红字部分；