PostgreSQL-9-別名與動態表復制
1、列別名
SELECT column_name AS alias_name FROM table_name conditions... ;
alias_name: 它指定分配給列的臨時名稱
SELECT name,MAX(salary) AS salary_max FROM company3 GROUP BY name; 將salary的最大值設置為別名salary_max
SELECT age,classno,studentname AS n FROM student; 將studentname設置為別名n
2、表別名
SELECT column1, column2.... FROM table_name AS alias_name conditions....;
alias_name：它指定分配給表的臨時名稱。
SELECT e.id,e.name,e.salary,d.dept
FROM employees AS e, department AS d
WHERE e.id = d.id; 這裏設置了company3和department兩個表的別名c和d，簡化代碼
SELECT * FROM employees,department
WHERE employees.id = department.id; 根據字段連接數據
3、動態表復制
\h CREATE TABLE AS
\h CREATE TABLE LIKE
查看語句說明 → 創建表 + 填充表
ALTER TABLE student ADD CONSTRAINT pkey PRIMARY KEY (no); 設置student表格的主鍵
ALTER TABLE student ADD CONSTRAINT agecheck CHECK (age > 0); 設置student表格的CHECK約束
ALTER TABLE student ALTER COLUMN studentname SET NOT NULL; 設置字段非空約束
CREATE TABLE student2 AS SELECT studentname,age FROM student WHERE age <15;
\d student 查看student的表格參數
\d student2 查看student2的表格參數
註意：CREATE TABLE AS復制出來的表，所有約束、註釋和序列都沒有被拷貝，但數據成功拷貝
CREATE TABLE student3 (LIKE student);
SELECT * FROM student3;
\d student 查看student的表格參數
\d student3 查看student2的表格參數
註意：CREATE TABLE LIKE和CREATE TABLE AS不同的是，復制成功拷貝了所有NOT-NULL約束，並且沒有拷貝表數據
PostgreSQL-9-別名與動態表復制