update_statement ::=
UPDATE {table_reference | [THE] (subquery1)} [alias]
 SET {
        column_name = {sql_expression | (subquery2)}
      | (column_name [, column_name]...) = (subquery3)} 
        [, {column_name = {sql_expression | (subquery2)}
      | (column_name [, column_name]...) = (subquery3)
     }]...
 [WHERE {search_condition | CURRENT_OF cursor_name}] [returning_clause]