PostgreSQL左外连接 LEFT OUTER JOIN

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。
如下图中所表示：
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语法：文章源自菜鸟学院-https://www.cainiaoxueyuan.com/sjk/4901.html

SELECT table1.columns, table2.columns  
FROM table1  
LEFT OUTER JOIN table2  
ON table1.common_filed = table2.common_field;

示例

看这个例子，现在看看下面一个表1 - “EMPLOYEES”，具有以下数据。文章源自菜鸟学院-https://www.cainiaoxueyuan.com/sjk/4901.html

文章源自菜鸟学院-https://www.cainiaoxueyuan.com/sjk/4901.html

表2： DEPARTMENT文章源自菜鸟学院-https://www.cainiaoxueyuan.com/sjk/4901.html

创建另一个表“DEPARTMENT”并插入以下值。文章源自菜鸟学院-https://www.cainiaoxueyuan.com/sjk/4901.html

-- 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);

现在，DEPARTMENT有以下数据 -
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执行以下左连接查询：文章源自菜鸟学院-https://www.cainiaoxueyuan.com/sjk/4901.html

SELECT EMPLOYEES.ID, EMPLOYEES.NAME, DEPARTMENT.DEPT  
FROM EMPLOYEES 
LEFT OUTER JOIN DEPARTMENT  
ON EMPLOYEES.ID = DEPARTMENT.ID;

得到以下结果 -文章源自菜鸟学院-https://www.cainiaoxueyuan.com/sjk/4901.html

文章源自菜鸟学院-https://www.cainiaoxueyuan.com/sjk/4901.html

从上面图中可以看到，左表(EMPLOYEES)全部列出来，而右表(DEPARTMENT)没有匹配上的项全留为空值。文章源自菜鸟学院-https://www.cainiaoxueyuan.com/sjk/4901.html