如何使用內部聯接查詢多個表？

Question

給定表創建SQL和插入SQL，如何生成查詢以特定方式顯示信息？

CREATE TABLE cities (
   id serial NOT NULL UNIQUE PRIMARY KEY,
   iname varchar(100) NOT NULL UNIQUE
)

CREATE TABLE suburbs (
    id serial NOT NULL UNIQUE PRIMARY KEY,
    icity integer REFERENCES cities (id), 
    iname varchar(100) NOT NULL UNIQUE
)
CREATE TABLE type (
   id serial NOT NULL UNIQUE PRIMARY KEY,
   iname varchar(100) NOT NULL UNIQUE
)
CREATE TABLE sale (
   id serial NOT NULL UNIQUE PRIMARY KEY,
   iname varchar(100) NOT NULL UNIQUE
)

CREATE TABLE estate (
    id serial NOT NULL UNIQUE PRIMARY KEY,
    icity integer REFERENCES cities (id),
    isuburb integer REFERENCES suburbs (id),
    itype integer REFERENCES type (id),
    isale integer REFERENCES sale (id),
    idescription text,
    itimestamp timestamp DEFAULT CURRENT_TIMESTAMP
)

INSERT INTO cities (iname) VALUES ('Johannesburg');
INSERT INTO suburbs (icity, iname) VALUES (1, 'Westbury');
INSERT INTO type (iname) VALUES ('Room');
INSERT INTO sale (iname) VALUES ('Rent');

INSERT INTO estate (icity, isuburb, itype, isale, idescription) VALUES (1, 1, 1, 1, 'A Nice place in a real bad neighbor hood');

現在，我希望表屬性中的數字值由它們表示的字符串值顯示。

約翰內斯堡EG 1號酒店，Westbury，客房，租金，描述

SQL Query的用途是什么，我更關心使用postgreSQL。

Answer 1

你可以嘗試像

select * from table1  
inner join table2 on tabl1.pk = table2.FK 
inner join table3 on tabl1.pk = table.FK 

最后

select table2.iname,table3.iname,table4.iname,table1.idescription 
from estate  as table1 
inner join sale  as table2 on table1.isale = table2.id 
inner join  type  as table3 on table1.itype  = table3.id 
inner join  suburbs  as table3 on table1.isuburb = table3.id 
inner join  cities as table4 on table1.icity = table4.id 

如果要獲取有關聯接的信息，請查看下圖

回答： 如何確定何時使用右連接/左連接或內部連接，或者如何確定哪張表在哪一邊？

Answer 2

首先，它們必須具有某種共同的領域。 假設它們之間的公共字段稱為<tablename>_ID ； 您的操作方式如下：

select A.colx, A.coly, A.colz, B.colx, B.colw, c.cold 
from A inner join B on A.ID=B.A_ID 
     inner join C on C.A_ID=A.ID

Answer 3

select suburbs.icity
        , cities.iname
        , suburbs.iname
        , type.iname
        , sale.iname
        , estate.idescription
from estate
inner join suburbs on suburbs.id = estate.isuburb
inner join cities on cities.id = estate.icity
inner join type on type.id = estate.itype
inner join sale on sale.id = estate.isale

Answer 4

通常，您正在尋找聯接。

select ct.iname as CityName,sb.name as SuburbName,et.*
from estate et
join cities ct on ct.id=et.icity
join suburbs sb on sb.id=et.isuburb

等等...

Answer 5

你可以這樣：

SELECT *
FROM A
INNER JOIN B 
ON B.id = A.b
INNER JOIN C
ON C.id = A.c

Answer 6

根據您的要求檢查此示例。

SELECT A.*,B.*,C.*
FROM A , B, C
WHERE C.c = B.b
  AND B.b = A.a

Answer 7

 -> select * from estate
 -> inner join cities on estate.icity=cities.id
 -> inner join suburbs on estate.isuburb=suburbs.id
 -> inner join type on estate.itype=type.id
 -> inner join sale on estate.isale=sale.id;