单表的MySQL UPDATE语句：
UPDATE [LOW_PRIORITY] [IGNORE] tbl_name
SET col_name1=expr1 [, col_name2=expr2 ...]
[WHERE where_definition]
[ORDER BY ...]
[LIMIT row_count]
多表的UPDATE语句：
UPDATE [LOW_PRIORITY] [IGNORE] table_references
SET col_name1=expr1 [, col_name2=expr2 ...]
[WHERE where_def
