Functions, John Shepherd 第一讲

example 1

create or replace function
          add2a(a integer,b integer) returns integer
as $$
select a + b;
$$ language 'sql';

This function is immutable(without any access to database)
函数名 add2a,函数参数确定,a integer

注意：integer放在变量名 a 后面

返回值设置：returns [data type]
as $$具体的函数执行语句，在此为一句sql select$$
末尾language设定：此处as中的具体操作仅涉及到sql

example 2

--immutable
--Function Returns a  tuple
create type Pair as (x integer, y integer)
//创建一个新类型,长度为2的元组
create or replace function 
        mkpair(a integer, b integer)returns Pair
as $$
declare
        p Pair;
begin
        p.x := a;
        p.y := b;
        returns p;
end;
$$ language 'plpgsql';

declare语句： declare 变量名变量类型;
begin --- end: 初步理解类似于一个transaction，之后再回来修改、确认
:= 赋值符号

example 3

--function returns a set of atomic values
create or replace function
          seq(hi integer) returns setof integer
as $$
declare
        i  integer;
begin
        i := 1;
        while (i<=hi) loop
            returns next i;
            i := i+1;
        end loop;
        return; 
end;
$$ language 'plpgsql';

while loop --- end loop;
loop与end loop;是while的括号，由于

PLpgSQL是一门old fashioned language

returns next i;

每次返回一个i

setof ...

return possibly more than one row

image.png

example 4 练习

create or replace function
            squares(top integer) returns setof integer
as $$
declare 
        i integer;
begin
        i := 1;
        while (i<=top) loop
            returns next i*i;
            i:=i+1;
        end loop;
        returns;
end;
$$ language 'plpgsql';

John Shepherd 的代码：

create or replace type Pair as (x integer, y integer);
create or replace function
                squares(top integer) returns setof integer
as  $$
        declare
            i integer;
            p Pair;
        begin
            for i in 1..top loop
              p.x = i
              p.y= i*i
              return next p;
              end loop;
              return;
         end;
$$ language 'plpgsql';

for循环的用法：
for variable in start..end, both start and end are inclusive.

image.png

example 5 开始读数据库了

--function read from database
-- and returns a set of atomic values
create or replace function
          pizzaWith(_topping text) returns setof text
as $$
declare
        _p Pizzas;
begin
        for _p in 
              select p.*
              from     Pizza p
                       join Has h on h.pizza = p.id
                       join Topping t  on h.topping = t.id
              where  t.name = _topping
        loop
              return next  _p.name;
        end loop;
end;
$$ language 'plpgsql';