1. Additional video

1.1 Aggregate

--Aggregate to count all values (even nulls)

CREATE FUNCTION
    oneMore(sum int, x integer) RETURNS integer
AS $$
BEGIN
    if x is null then
        return sum + 1;
    else
        return sum + 1;
    end if;
END;
$$ LANGUAGE plpgsql;

CREATE AGGREGATE countAll(integer)(
    stype = integer, --the accumulator type
    initcond = 0, --initial accumulator value
    sfunc = oneMore --newstate function
); 



--pizza example
SELECT p.name as pizza, list(t.name)
FROM Pizzas p
    JOIN Has h ON p.id = h.pizza
    JOIN Toppings t ON t.di = h.Toppings
GROUP BY p.name;
--aggregate to concat strings, using '|' as separator
CREATE OR REPLACE FUNCTION 
    append(soFar text, item text) RETURNS text
AS $$
BEGIN
    if soFar = '' then
        return item;
    else 
        return soFar||'|'||item;
    end if;
END;
$$ LANGUAGE plpgsql

DROP AGGREGATE if EXISTS list(text);  --重要！！

CREATE AGGREGATE list(text)(
    stype = text, --the accumulator type
    initcond = '', --initial accumulator value
    sfunct = append --newstate function
);

1.2 Trigger

--Maintain count of pizzas available in each store

drop trigger if exists maintainVcount on SoldIn;

create or replace function
    changeVarieties() returns trigger
as $$
begin
    if TG_OP = 'INSERT' then
        update Stores
        set varieties = varieties + 1
        where id = NEW.stroe;
    elsif TG_OP = 'DELETE' then
        update Stores
        set varieties = varieties - 1
        where id = OLD.store;
    else
        raise exception 'changeVarieties() failed';
    end if;
    return null;
end;
$$ language plpgsql;


create trigger maintainVcount 
after insert or delete on SoldIn
for each row
execute procedure changeVarieties();

2. Trigger

PostgreSQL的syntax如下：

CREATE TRIGGER TriggerName
{AFTER|BEFORE} Event1 [OR Event2 ...]
--After一般用于update，before用于insert
[ WHEN (Condition)]
FOR EACH {ROW|STATEMENT}
--Row每次都修改，Statement是所有执行完后统一修改。假设有1million数据要修改，最好用statement，是在trigger运行结束后统一操作，而row的话每条数据都要执行一次，效率较低。
EXECUTE PROCEDURE FunctionName(args ...);

2.1 例1

Consider a database of people in the USA, ensure that only valid state codes are used.

CREATE TABLE Person(
  id integer,
  ssn varchar(11) unique,
  ...
  state char(2),
  primary key (id)
);
CREATE TABLE States(
  id integer,
  code char(2) unique,
  ...
  primary key (id)
);

TRIGGER:

CREATE FUNCTION CheckState() RETURNS TRIGGER
AS $$
BEGIN
  new.state = upper(trim(new.state))
  --trim()用来去除多余杂项，这里去除的是空格，之后转换为upper case
  if (new.state !~ '^[A-Z][A-Z]$') then
    raise exception 'Code must be two alpha chars';
    --检查格式是否正确
  end if;
  select * from States where code = new.state;
  --检查state是否在state的table中，是否legal
  if (not found) then
    raise exception 'Invalid code %', new.state;
    --用%来替代后面的variable，这里是new.state
  end if;
  return new;
END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER CheckState BEFORE insert or update
ON Person for each ROW EXECUTE PROCEDURE checkstate();

2.2 例2

department salary totals changes with individual salary change.
需要保证下面的情况成立

CREATE ASSERTION TotalSalary CHECK(
  not exists (
    select d.id from Department d
    where d.totSal <>
      (select sum(e.salary)
      from Employee e
      where e.dept = d.id)
  )
)

TRIGGER1: new employees arrive

CREATE FUNCTION totalSalary1() RETURNS TRIGGER
AS $$
BEGIN
  if (new.dept is not null) then
    update Department
    set totSal = totSal + new.salary
    where Department.id = new.dept;
  end if;
  return new;
END;
$$ LANGUAGE plpgsql;

create trigger TotalSalary1 
after insert on Employees
for each row execute procedure totalSalary1();

TRIGGER2: employees change departments/salaries

create function totalSalary2() returns trigger
as $$
begin
  update Department
  set totSal = totSal + new.salary
  where Department.id = new.dept;
  update Department
  set totSal = totSal - old.salary
  where Department.id = old.dept;
  return new;
end;
$$ language plpgsql;

create trigger TotalSalary2 
after update on Employee
for each row execute procedure totalSalary2();

TRIGGER3: employees leave

create function totalSalary3() returns trigger
as $$
begin
  if (old.dept is not null) then
    update Department
    set totalSal = totalSal - old.salary
    where Department.id = old.dept;
  end if;
  return old;
end;
$$ language plpgsql;

create trigger TotalSalary3
after delete on Employee
for each row execute procedure totalSalary3();

3. Database Programming

大多数常见语言都可以操作数据库，如C的libpq，Java的JDBC(Java Database Connectivity)。常见的调用操作数据库的结构如下：

db = connect_to_dbms(DBname, User/Password);
query = build_SQL("SqlStatementTemplate", values);
results = execute_query(db, query);
while (more_tuples_in(results))
{
  tuple = fetch_row_from(results);
  ...
}

以PHP为例：

$db = dbConnect("dbname = myDB");
...
$query = "select a,b,c from R where c >= %d";
$result = dbQuery($db, mkSQL($query, $min);
while ($tuple = dbNext($result)){
  $tmp = $tuple["a"] - $tuple["b"] - $tuple["c"]
  ...
  list($a, $b, $c) = $tuple;
  $tmp = $a - $b - $c;
}
...