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第4章. 简单的查询
4.1.查询的基本结构
select [distinct | all] column1, column2
from table_name
[where search_condition]
[group by group_by_expression]
[having search_condition]
[order by order_expression [asc|desc]]
➢ distinct:去掉重复的行 select distinct column from table,distinct 后面必须跟列的名字才可以,
如果是 distinct *则不起作用。默认是 ALL。
➢ having:group by 的补充,另外的行选择标准,以便根据 group by 子句产生的结果进行筛选。
SELECT 执行顺序 FROM-WHERE-GROUP BY- HAVING –SELECT-ORDER BY
4.1.1. 排序
Order by 一般放在所有子句的后面,其中后面可以跟多列,多列时,只有前面的列出现重复数据时才采用
第二个列排序,最后加 DESC
例子:
select column1, column2 from table_name order by column1, column2
select column1, column2 from table_name order by 1, 2
select * from table_name where column >= ‘40’ order by column
4.2.where 单值较运算符
select * from table_name where age between 30 and 50,对于字符也适用。