Mysql自学

Posted 2022-01-31 爱唱歌的猫★

sql自学

第三次
目标：掌握单表查询的定义，掌握SELETE
一,单表查询定义
从一个数据表中查询一列或多列数据
种类：
①:单表查询：SELECT [列名] FROM [表名]
②:多表查询：SELECT [列名1] [列名2] [列名3] FROM [表名]
③:所有列查询：SELECT * FROM [表名]
例:
①SELECT prod_id FROM milk_tea;
②SELECT prod_name FROM milk_tea;
③SELECT prod_id,prod_name FROM milk_tea;
④SELECT * FROM milk_tea;
二,结果展示：SELECT
1.别名设置-AS
为指定查询列设置别名
SELECT [列名1] [列名2] AS [列名] [列名3] FROM [表名]
适合场景：当前列名不符合规定，重新命名
新生成的列
例：
①SELECT prod_id,prod_name,net_w AS net_weight,volid_month AS ‘保质期’ FROM milk_tea;(对列名使用别名)
②SELECT m.prod_id,m.prod_name FROM milk_tea AS m;(对表名使用别名)
2.常熟添加
在结果中添加常数列
SELECT ‘常数’ AS [别名] FROM [表名];
适合场景：添加特定值
例：
①:SELECT prod_id,prod_name,0.9 AS discount FROM milk_tea;
②:SELECT prod_id,prod_name,‘零食’ AS class FROM milk_tea;
★如果不设置别名则将文本默认为别名
3.四则运算
列与列之间，列与常熟之间
SELECT [四则运算表达式] FROM [表名];
适合场景:需要一定的运算
例
①SELECT m.prod_id,m.in_price,m.sale_price,m.sale_price-m.in_price AS profit FROM milk_tea AS m;
②SELECT m.prod_id,m.in_price,m.sale_price,m.sale_price*0.9 AS new_price FROM milk_tea AS m;
★均不改变原表，只是将结果展现的修饰