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]
 [row_expression [, row_expression]... INTO 
 {variable_name | :host_variable_name} 
 [, {variable_name | :host_variable_name}]...];