把具有相同字段的纪录删除，只留下一条。

例如，表test里有id,name字段

如果有name相同的记录 只留下一条，其余的删除。

name的内容不定，相同的记录数不定。-The record has the same field of deleted, leaving only one.

For example, table test has id, name field

If you have the same name, leaving only a record, delete the rest.

name of variable content, the same number of records uncertain.