PostgreSQL的使用向导

文献参考:https://www.postgresql.org/docs/12/tutorial.html

这个文档，基本对PG的使用有一个感性认识。

数据库

创建数据库

[postgres@db ~]$ createdb zsddb

进入数据库

$ psql zsddb

查看版本

zsddb=# SELECT version();
                                                 version                                                 
---------------------------------------------------------------------------------------------------------
 PostgreSQL 12.1 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.4.7 20120313 (Red Hat 4.4.7-23), 64-bit
(1 row)

查看当前时间日期

zsddb=# SELECT current_date;
 current_date 
--------------
 2019-12-02
(1 row)

简单的select

zsddb=# SELECT 2 + 2;
 ?column? 
----------
        4
(1 row)

获得帮助命令

zsddb=# \h

退出psql客户端

两种方式，如下:
zsddb=# quit
zsddb=# \q

创建表

weather和cities表的创建

CREATE TABLE weather (
    city            varchar(80),
    temp_lo         int,           -- low temperature
    temp_hi         int,           -- high temperature
    prcp            real,          -- precipitation
    date            date
);



CREATE TABLE cities (
    name            varchar(80),
    location        point
);

删除表

DROP TABLE tablename;

插入数据

INSERT INTO weather VALUES ('San Francisco', 46, 50, 0.25, '1994-11-27');

The point type requires a coordinate pair as input, as shown here:

INSERT INTO cities VALUES ('San Francisco', '(-194.0, 53.0)');


INSERT INTO weather (city, temp_lo, temp_hi, prcp, date)
    VALUES ('San Francisco', 43, 57, 0.0, '1994-11-29');

INSERT INTO weather (date, city, temp_hi, temp_lo)
    VALUES ('1994-11-29', 'Hayward', 54, 37);

数据库导出成csv文本文件

zsddb=# copy weather to '/home/postgres/weather.csv' delimiter ',' csv header;
COPY 3
zsddb=# exit
[postgres@db ~]$ cat /home/postgres/weather.csv 
city,temp_lo,temp_hi,prcp,date
San Francisco,46,50,0.25,1994-11-27
San Francisco,43,57,0,1994-11-29
Hayward,37,54,,1994-11-29

查询表的语句

查询所有的数据

zsddb=# SELECT * FROM weather;
     city      | temp_lo | temp_hi | prcp |    date    
---------------+---------+---------+------+------------
 San Francisco |      46 |      50 | 0.25 | 1994-11-27
 San Francisco |      43 |      57 |    0 | 1994-11-29
 Hayward       |      37 |      54 |      | 1994-11-29
(3 rows)

查看指定列

zsddb=# SELECT city, temp_lo, temp_hi, prcp, date FROM weather;
     city      | temp_lo | temp_hi | prcp |    date    
---------------+---------+---------+------+------------
 San Francisco |      46 |      50 | 0.25 | 1994-11-27
 San Francisco |      43 |      57 |    0 | 1994-11-29
 Hayward       |      37 |      54 |      | 1994-11-29
(3 rows)

查看平均温度

zsddb=# SELECT city, (temp_hi+temp_lo)/2 AS temp_avg, date FROM weather;
     city      | temp_avg |    date    
---------------+----------+------------
 San Francisco |       48 | 1994-11-27
 San Francisco |       50 | 1994-11-29
 Hayward       |       45 | 1994-11-29
(3 rows)

查看城市是'San Francisco',降水率大于0的数据。

zsddb=# SELECT * FROM weather
zsddb-#     WHERE city = 'San Francisco' AND prcp > 0.0;
     city      | temp_lo | temp_hi | prcp |    date    
---------------+---------+---------+------+------------
 San Francisco |      46 |      50 | 0.25 | 1994-11-27
(1 row)

对城市排序

zsddb=# SELECT * FROM weather
zsddb-#     ORDER BY city;
     city      | temp_lo | temp_hi | prcp |    date    
---------------+---------+---------+------+------------
 Hayward       |      37 |      54 |      | 1994-11-29
 San Francisco |      46 |      50 | 0.25 | 1994-11-27
 San Francisco |      43 |      57 |    0 | 1994-11-29
(3 rows)

城市排序后，对温度再排序

zsddb=# SELECT * FROM weather
zsddb-#     ORDER BY city, temp_lo;
     city      | temp_lo | temp_hi | prcp |    date    
---------------+---------+---------+------+------------
 Hayward       |      37 |      54 |      | 1994-11-29
 San Francisco |      43 |      57 |    0 | 1994-11-29
 San Francisco |      46 |      50 | 0.25 | 1994-11-27

城市去重DISTINCT

zsddb=# SELECT DISTINCT city
zsddb-#     FROM weather;
     city      
---------------
 Hayward
 San Francisco
(2 rows)

去重+排序

SELECT DISTINCT city
    FROM weather
    ORDER BY city;

表连接查询

查看zsddb数据库下的表名称

zsddb=# \dt
          List of relations
 Schema |  Name   | Type  |  Owner   
--------+---------+-------+----------
 public | cities  | table | postgres
 public | weather | table | postgres
(2 rows)

查看表结构

zsddb=# \d weather
Table "public.weather"
 Column  |         Type          | Collation | Nullable | Default 
---------+-----------------------+-----------+----------+---------
 city    | character varying(80) |           |          | 
 temp_lo | integer               |           |          | 
 temp_hi | integer               |           |          | 
 prcp    | real                  |           |          | 
 date    | date                  |           |          | 

zsddb=# \d cities
Table "public.cities"
  Column  |         Type          | Collation | Nullable | Default 
----------+-----------------------+-----------+----------+---------
 name     | character varying(80) |           |          | 
 location | point                 |           |          | 

weather.city和cities.name做表连接查询。

zsddb=# SELECT *
zsddb-#     FROM weather, cities
zsddb-#     WHERE city = name;
     city      | temp_lo | temp_hi | prcp |    date    |     name      | location  
---------------+---------+---------+------+------------+---------------+-----------
 San Francisco |      46 |      50 | 0.25 | 1994-11-27 | San Francisco | (-194,53)
 San Francisco |      43 |      57 |    0 | 1994-11-29 | San Francisco | (-194,53)
(2 rows)

一般来说，你可能会输出你想要的列而不是"*"

zsddb=# SELECT city, temp_lo, temp_hi, prcp, date, location
zsddb-#     FROM weather, cities
zsddb-#     WHERE city = name;
     city      | temp_lo | temp_hi | prcp |    date    | location  
---------------+---------+---------+------+------------+-----------
 San Francisco |      46 |      50 | 0.25 | 1994-11-27 | (-194,53)
 San Francisco |      43 |      57 |    0 | 1994-11-29 | (-194,53)

虽然pg的parser会自动分析city和name是属于哪个表的列。但是如果有同名的列话，还需手动指定，如下:

zsddb=# SELECT weather.city, weather.temp_lo, weather.temp_hi,
zsddb-#        weather.prcp, weather.date, cities.location
zsddb-#     FROM weather, cities
zsddb-#     WHERE cities.name = weather.city;
     city      | temp_lo | temp_hi | prcp |    date    | location  
---------------+---------+---------+------+------------+-----------
 San Francisco |      46 |      50 | 0.25 | 1994-11-27 | (-194,53)
 San Francisco |      43 |      57 |    0 | 1994-11-29 | (-194,53)
(2 rows)

上述的写法被广泛认为是good style ，还有一种替代写法如下:

zsddb=# SELECT *
zsddb-#     FROM weather INNER JOIN cities ON (weather.city = cities.name);
     city      | temp_lo | temp_hi | prcp |    date    |     name      | location  
---------------+---------+---------+------+------------+---------------+-----------
 San Francisco |      46 |      50 | 0.25 | 1994-11-27 | San Francisco | (-194,53)
 San Francisco |      43 |      57 |    0 | 1994-11-29 | San Francisco | (-194,53)
(2 rows)

其实为了说明，上面的连接其实是inner join的表连接。

现在我们要把 Hayward那条记录找回来，我们需要对weather 表中每条记录与cities的表的记录相匹配，如果
匹配不到，就用"empty value"来显示，这个就需要用到outer join。如下:

zsddb=# SELECT *
zsddb-#     FROM weather LEFT OUTER JOIN cities ON (weather.city = cities.name);
     city      | temp_lo | temp_hi | prcp |    date    |     name      | location  
---------------+---------+---------+------+------------+---------------+-----------
 San Francisco |      46 |      50 | 0.25 | 1994-11-27 | San Francisco | (-194,53)
 San Francisco |      43 |      57 |    0 | 1994-11-29 | San Francisco | (-194,53)
 Hayward       |      37 |      54 |      | 1994-11-29 |               | 
(3 rows)

当然还有right outer joins and full outer joins，这里不演示了。

我们还可以对表自身做一个表连接，叫做self join,如下:

zsddb=# SELECT W1.city, W1.temp_lo AS low, W1.temp_hi AS high,
zsddb-#     W2.city, W2.temp_lo AS low, W2.temp_hi AS high
zsddb-#     FROM weather W1, weather W2
zsddb-#     WHERE W1.temp_lo < W2.temp_lo
zsddb-#     AND W1.temp_hi > W2.temp_hi;
     city      | low | high |     city      | low | high 
---------------+-----+------+---------------+-----+------
 San Francisco |  43 |   57 | San Francisco |  46 |   50
 Hayward       |  37 |   54 | San Francisco |  46 |   50
(2 rows)

当然，以后通过对表一个别名，减少select打query的数量，如下:

zsddb=# SELECT *
zsddb-#     FROM weather w, cities c
zsddb-#     WHERE w.city = c.name;
     city      | temp_lo | temp_hi | prcp |    date    |     name      | location  
---------------+---------+---------+------+------------+---------------+-----------
 San Francisco |      46 |      50 | 0.25 | 1994-11-27 | San Francisco | (-194,53)
 San Francisco |      43 |      57 |    0 | 1994-11-29 | San Francisco | (-194,53)

Aggregate Functions 聚合函数

天气最大值

zsddb=# SELECT max(temp_lo) FROM weather;
 max 
-----
  46
(1 row)

如果想知道temp_lo最大值对应的城市

zsddb=# SELECT city FROM weather
zsddb-#     WHERE temp_lo = (SELECT max(temp_lo) FROM weather);
     city      
---------------
 San Francisco
(1 row)

使用聚合函数，需要group by来进行分组。

zsddb=# SELECT city, max(temp_lo)
zsddb-#     FROM weather
zsddb-#     GROUP BY city;
     city      | max 
---------------+-----
 Hayward       |  37
 San Francisco |  46
(2 rows)

聚合函数，需要group by，再加where条件，挑选出小于40的。

zsddb=# SELECT city, max(temp_lo)
zsddb-#     FROM weather
zsddb-#     GROUP BY city
zsddb-#     HAVING max(temp_lo) < 40;
  city   | max 
---------+-----
 Hayward |  37
(1 row)

update语句的应用

zsddb=# SELECT * FROM weather;
     city      | temp_lo | temp_hi | prcp |    date    
---------------+---------+---------+------+------------
 San Francisco |      46 |      50 | 0.25 | 1994-11-27
 San Francisco |      43 |      57 |    0 | 1994-11-29
 Hayward       |      37 |      54 |      | 1994-11-29
(3 rows)

zsddb=# UPDATE weather
zsddb-#     SET temp_hi = temp_hi - 2,  temp_lo = temp_lo - 2
zsddb-#     WHERE date > '1994-11-28';
UPDATE 2
zsddb=# SELECT * FROM weather;
     city      | temp_lo | temp_hi | prcp |    date    
---------------+---------+---------+------+------------
 San Francisco |      46 |      50 | 0.25 | 1994-11-27
 San Francisco |      41 |      55 |    0 | 1994-11-29
 Hayward       |      35 |      52 |      | 1994-11-29

delete语句的应用

zsddb=# SELECT * FROM weather;
     city      | temp_lo | temp_hi | prcp |    date    
---------------+---------+---------+------+------------
 San Francisco |      46 |      50 | 0.25 | 1994-11-27
 San Francisco |      41 |      55 |    0 | 1994-11-29
 Hayward       |      35 |      52 |      | 1994-11-29
(3 rows)

zsddb=# DELETE FROM weather WHERE city = 'Hayward';
DELETE 1
zsddb=# SELECT * FROM weather;
     city      | temp_lo | temp_hi | prcp |    date    
---------------+---------+---------+------+------------
 San Francisco |      46 |      50 | 0.25 | 1994-11-27
 San Francisco |      41 |      55 |    0 | 1994-11-29
(2 rows)

视图

CREATE VIEW myview AS
    SELECT city, temp_lo, temp_hi, prcp, date, location
        FROM weather, cities
        WHERE city = name;

zsddb=# SELECT * FROM myview;
     city      | temp_lo | temp_hi | prcp |    date    | location  
---------------+---------+---------+------+------------+-----------
 San Francisco |      46 |      50 | 0.25 | 1994-11-27 | (-194,53)
 San Francisco |      41 |      55 |    0 | 1994-11-29 | (-194,53)
(2 rows)

事务

PostgreSQL通过BEGIN和COMMIT这一对代表事务的开启。

BEGIN;
UPDATE accounts SET balance = balance - 100.00
    WHERE name = 'Alice';
-- etc etc
COMMIT;