sql 等额本息

/* 等额本息计算还款额
  a 贷款总额
  b 月利率
  m 贷款月数
*/
execute block
returns (result double precision) as
declare a int = ; -- 单位 = 万
declare b double precision = 4.9;
declare m int = ;
begin
    :a = :a * ;
    :b = :b /100.000/12.000;
    select :a * :b * power(( + :b), :m) / (power(( + :b), :m) - ) from RDB$DATABASE
    into :result;
    suspend;
end