今天学习表的增删改相关内容，以下为课后作业打卡；

一.创建表products

CREATE TABLE products
( product_id integer,   
  product_name char(30),    
  category char(20)
);
COMMENT ON COLUMN products.product_id IS '产品编号';
COMMENT ON COLUMN products.product_name IS '产品名字';
COMMENT ON COLUMN products.category IS '种类';

二.向表中插入数据

1、一次插入一条

INSERT INTO products VALUES (1502, 'olympus camera', 'electrncs');

2、一次插入多条

INSERT INTO products VALUES 
(1601, 'lamaze', 'toys'),
(1700, 'wait interface', 'Books'),
(1666, 'harry potter', 'toys');

三.获取指定数量的记录

1、查询表中一条记录

omm=# select * from products limit 1;
omm=# product_id | product_name | category
------------±-------------------------------±---------------------
1502 | olympus camera | electrncs
(1 row)

2、查询表中三条记录

select * from products limit 3;
product_id | product_name | category
------------±-------------------------------±---------------------
1502 | olympus camera | electrncs
1601 | lamaze | toys
1700 | wait interface | Books
(3 rows)

3、查询表中所有记录

omm=# select * from products;
product_id | product_name | category
------------±-------------------------------±---------------------
1502 | olympus camera | electrncs
1601 | lamaze | toys
1700 | wait interface | Books
1666 | harry potter | toys
(4 rows)

四.按指定条件更新记录

将满足product_id > 1600的记录的product_id更新为product_id – 1000，并查看products中所有记录是否更新成功

omm=# update products set product_id=product_id-1000 where product_id > 1600;
UPDATE 0
omm=# select * from products;
 product_id |          product_name          |       category       
------------+--------------------------------+----------------------
       1502 | olympus camera                 | electrncs           
        601 | lamaze                         | toys                
        700 | wait interface                 | Books               
        666 | harry potter                   | toys                
(4 rows)

从结果可以看到除了1502的记录其余3条记录product_id的值均已减去1000；

五.删除category为toys的所有记录

omm=# delete from products where category='toys';
DELETE 2
omm=# select * from products;
 product_id |          product_name          |       category       
------------+--------------------------------+----------------------
       1502 | olympus camera                 | electrncs           
        700 | wait interface                 | Books               
(2 rows)

查询表中所有数据，已经没有toys的记录；

六.删除products中所有数据

omm=# delete from products;
DELETE 2
omm=# select * from products;
 product_id | product_name | category 
------------+--------------+----------
(0 rows)

查询结果 0 rows，表中已经没有数据；

七.删除表products

drop table products;