列表列

Question

想象这些表结构

组织表

===================== 
| id |  name  | 
-----+----------------+ 
| 1 | Organization 1 | 
===================== 

代替表

================================= 
| id | name | organization_id | 
-----+----------+-----------------+ 
| 1 | Place 1 |  1  | 
-----+----------+-----------------+ 
| 2 | Place 2 |  1  | 
-----+----------+-----------------+ 
| 3 | Place 3 |  1  | 
================================= 

档案表

============= 
| username | 
-------------- 
| 111@xxx.com | 
-------------- 
| 222@xxx.com | 
============= 

Visitedplace表

================================================ 
| id | place_id | profile_username | visiteddate | 
-----+----------+---------------- -+-------------+ 
| 1 |  1 | 111@xxx.com | 2017-01-01 | 
-----+----------+------------------+-------------+ 
| 2 |  2 | 111@xxx.com | 2017-02-01 | 
-----+----------+------------------+-------------+ 
| 3 |  1 | 111@xxx.com | 2017-01-15 | 
================================================ 

下一步是创建表和数据插入到它的SQL statments

-- ---------------------------- 
-- Table structure for organization 
-- ---------------------------- 
DROP TABLE IF EXISTS "organization"; 
CREATE TABLE "organization" (
"id" int4 NOT NULL, 
"name" varchar(255) COLLATE "default" NOT NULL 
) WITH (OIDS=FALSE); 
-- ---------------------------- 
-- Records of organization 
-- ---------------------------- 
BEGIN; 
INSERT INTO "organization" VALUES ('1', 'Organization 1'); 
COMMIT; 
-- ---------------------------- 
-- Table structure for place 
-- ---------------------------- 
DROP TABLE IF EXISTS "place"; 
CREATE TABLE "place" (
"id" int4 NOT NULL, 
"name" varchar(255) COLLATE "default" NOT NULL, 
"organization_id" int4 NOT NULL 
)WITH (OIDS=FALSE); 
-- ---------------------------- 
-- Records of place 
-- ---------------------------- 
BEGIN; 
INSERT INTO "place" VALUES ('1', 'Place 1', '1'); 
INSERT INTO "place" VALUES ('2', 'Place 2', '1'); 
INSERT INTO "place" VALUES ('3', 'Place 3', '1'); 
COMMIT; 
-- ---------------------------- 
-- Table structure for profile 
-- ---------------------------- 
DROP TABLE IF EXISTS "profile"; 
CREATE TABLE "profile" (
"username" varchar(255) COLLATE "default" NOT NULL 
)WITH (OIDS=FALSE); 
-- ---------------------------- 
-- Records of profile 
-- ---------------------------- 
BEGIN; 
INSERT INTO "profile" VALUES ('111@xxx.com'); 
INSERT INTO "profile" VALUES ('222@xx.com'); 
COMMIT; 
-- ---------------------------- 
-- Table structure for visitedplace 
-- ---------------------------- 
DROP TABLE IF EXISTS "visitedplace"; 
CREATE TABLE "visitedplace" (
"id" int4 NOT NULL, 
"place_id" int4 NOT NULL, 
"profile_username" varchar(255) COLLATE "default" NOT NULL, 
"visiteddate" date NOT NULL 
)WITH (OIDS=FALSE); 
-- ---------------------------- 
-- Records of visitedplace 
-- ---------------------------- 
BEGIN; 
INSERT INTO "visitedplace" VALUES ('1', '1', '111@xxx.com', '2017-02-24'); 
COMMIT; 
-- ---------------------------- 
-- Alter Sequences Owned By 
-- ---------------------------- 
-- ---------------------------- 
-- Primary Key structure for table organization 
-- ---------------------------- 
ALTER TABLE "organization" ADD PRIMARY KEY ("id"); 
-- ---------------------------- 
-- Primary Key structure for table place 
-- ---------------------------- 
ALTER TABLE "place" ADD PRIMARY KEY ("id"); 
-- ---------------------------- 
-- Primary Key structure for table profile 
-- ---------------------------- 
ALTER TABLE "profile" ADD PRIMARY KEY ("username"); 
-- ---------------------------- 
-- Primary Key structure for table visitedplace 
-- ---------------------------- 
ALTER TABLE "visitedplace" ADD PRIMARY KEY ("id"); 
-- ---------------------------- 
-- Foreign Key structure for table "place" 
-- ---------------------------- 
ALTER TABLE "place" ADD FOREIGN KEY ("organization_id") REFERENCES 
"organization" ("id") ON DELETE CASCADE ON UPDATE CASCADE; 
------------------------------ 
-- Foreign Key structure for table "visitedplace" 
-- ---------------------------- 
ALTER TABLE "visitedplace" ADD FOREIGN KEY ("profile_username") REFERENCES "profile" ("username") ON DELETE CASCADE ON UPDATE CASCADE; 
ALTER TABLE "visitedplace" ADD FOREIGN KEY ("place_id") REFERENCES "place" ("id 

") ON DELETE CASCADE ON UPDATE CASCADE; 

我的问题是

当我运行这下查询

select profile.username, count(distinct place.id) 
    from profile profile 
    left outer join visitedplace visitedplace on profile.username=visitedplace.profile_username 
    inner join place place on visitedplace.place_id=place.id 
    where place.organization_id=1 
    group by profile.username 

它返回一个结果

===================== 
| username | count | 
-------------+------- 
| 111@xxx.com | 2 | 
===================== 

但我期待下一个结果

===================== 
| username | count | 
-------------+------- 
| 111@xxx.com | 2 | 
-------------+------- 
| 222@xxx.com | 0 | 
===================== 

所以，我该怎么办查询返回我什么，我期待？

我使用Postgres的

Answer 1

用途使用left join。所以，所有连接都应该是外连接（在第一个之后）。而且，你必须要小心的where条款：

select p.username, count(distinct place.id) 
from profile p left outer join 
    visitedplace vp 
    on p.username = vp.profile_username left join 
    place pl 
    on vp.place_id = pl.id and pl.organization_id = 1 
group by p.username; 

如果你不小心，那么外连接变成一个inner join。