如何在另一個表中現有行的左連接上添加條件

Question

我有這個查詢：

SELECT
   p.id,
   COUNT(a.id) accounts,
   sum(case when a.state = '0' then 1 else 0 end) active_accounts
FROM Person p
  LEFT JOIN Account a
  ON a.person_id = p.id ADN 
WHERE p.id = 1
GROUP BY p.id;

如何通過將bank表上的account.bank_name = bank.name為accounts和active_accounts值來添加具有有效bank_name的帳戶條件？

這是數據庫方案：

Person (
  id
)

Account (
  id
  person_id
  state -- 0 - active, 1 - closed
  bank_name
)

Bank (
  id,
  name
)

如果“ Account bank_name不正確（“ Bank表中沒有這樣的行），則不應計算在內。

Answer 1

諸如此類，將左連接添加到bank表和where子句以檢查name是否為null：

SELECT
   p.id,
   COUNT(a.id) accounts,
   sum(case when a.state = '0' then 1 else 0 end) active_accounts
FROM Person p
  LEFT JOIN Account a ON
      a.person_id = p.id
  LEFT JOIN bank b ON
      a.bank_name = b.name
WHERE p.id = 1 and
    b.name is not null
GROUP BY p.id

Answer 2

參見http://sqlfiddle.com/#!9/2b43b3/3

 SELECT
   p.id,
   COUNT(a.id) accounts,
   sum(case when a.state = '0' then 1 else 0 end) active_accounts
FROM Person p
  LEFT JOIN Account a ON a.person_id = p.id   
  INNER JOIN Bank on a.bank_name = Bank.name 

WHERE p.id = 1
GROUP BY p.id;

使用外鍵，較精通的ddl會像這樣

create table person (
  id int unique not null
);

create table bank (
  id int unique not null,
  name nvarchar(10) 
);

create table account ( 
  id int unique not null, 
  person_id int,
  state bit, 
  bank_id int,
  FOREIGN KEY (bank_id) REFERENCES bank(id),
  FOREIGN KEY (person_id) REFERENCES person(id)
); 

Insert into person values (1), (2); 
insert into bank VALUES (1, "B1"),(2, "B2");
insert into account values (1,1,1,1), (2,1,0,1),(3,1,1,1),(4,2,1,1); 

這種DDL將允許您擁有沒有銀行的accounts （ bank_id == null）或具有有效的bank_ids accounts -DB會為您處理-不需要在ncharchar（）s上進行不安全的聯接-如果以下情況可能會出錯您提交一些尾隨/前導空格

Answer 3

CTE的另一種方式（如果您的數據庫支持）：

with account_valid as (
    select
        a.*
    from
        Account a 
        inner join bank b on
            a.bank_name = b.name
)
select
    p.id,
    count(a.id) accounts,
    sum(case when a.state = '0' then 1 else 0 end) active_accounts
from
    Person p
    left join account_valid a ON
      a.person_id = p.id ADN   
where
    p.id = 1 -- condition just for test
group by
    p.id

或使用嵌套查詢：

select
    p.id,
    count(a.id) accounts,
    sum(case when a.state = '0' then 1 else 0 end) active_accounts
from
    Person p
    left join (
        select
            a.*
        from
            Account a 
            inner join bank b on
                a.bank_name = b.name
    ) account_valid a ON
      a.person_id = p.id ADN   
where
    p.id = 1 -- condition just for test
group by
    p.id