字符串替换

REPLACE ( fieldname, search, replace )

数据去重

DELETE ...

DELETE FROM table_copy 
WHERE key IN ( 
    SELECT t.key FROM (
    SELECT key FROM table_copy GROUP BY key having count(*) >1
    ) t
) 
AND id NOT IN(
    SELECT t.id FROM (
    SELECT id FROM table_copy GROUP BY key having count(*) >1
    ) t
)