PostgreSQL左外连接 LEFT OUTER JOIN
外连接是内联的延伸,外连接有三种类型。它们分别如下 -
- 左外连接
- 右外连接
- 全外连接
左外连接
左外连接返回从“ON”条件中指定的左侧表中的所有行,只返回满足条件的另一个表中的行。
语法:
SELECT table1.columns, table2.columns
FROM table1
LEFT OUTER JOIN table2 ON table1.common_filed = table2.common_field;
实例数据:
表1: EMPLOYEES表
表2: DEPARTMENT
创建另一个表“DEPARTMENT”并插入以下值。
-- Table: public.department
-- DROP TABLE public.department;
CREATE TABLE public.department
(
id integer,
dept text,
fac_id integer
)
WITH (
OIDS=FALSE
);
ALTER TABLE public.department
OWNER TO postgres;
-- 插入数据
INSERT INTO department VALUES(1,'IT', 1);
INSERT INTO department VALUES(2,'Engineering', 2);
INSERT INTO department VALUES(3,'HR', 7);
执行以下左连接查询:
SELECT EMPLOYEES.ID, EMPLOYEES.NAME, DEPARTMENT.DEPT
FROM EMPLOYEES
LEFT OUTER JOIN DEPARTMENT ON EMPLOYEES.ID = DEPARTMENT.ID;