第六章 过滤数据

1.使用where语句:
select column_name_0 from table_name where column_name_1 = value;

select column_name_0 from table_name where column_name_1 = value order by column_name_2 desc;

2.where 子语句操作符:
= 
<>			不等
!=
<
<=
>
>=
between

select column_name_0 from table_name where column_1 between value_0 and value_1;

3.空值检查:
在创建表的时候，可以指定其中的列是否可以不包含值。在一个列不包含值时，称其为包含空值null，null 是无值的意思
is null 可以用来检查具有null值的列
select column_name_0 from table_name where column_name_1 is null;