SQL 第一课

1.      Select * from table

 

练习：select * from po_lines_all

 

2.      Select column1,column2 from table

 

 

练习：select last_update_date , po_header_id from po_lines_all

 

知识点：

 

•       SQLstatements are not case sensitive.

SQL语句是不分大小写的

•       SQLstatements can be on one or more lines.

SQL 语句可以在一行或者是多行

•       Keywordscannot be abbreviated or split across lines.

关键字不能缩写或是拆分

•      Clauses are usually placed on separate lines.

条款通常放在单独的线路上

•      Indents are used to enhance readability.

缩进为了增强可读性

3.      Arithmetic Expressions算数表达式

练习1：select last_update_date,unit_price,2 * (unit_price + 200)from po_lines_all

•      Multiplication and division take priorityover addition and subtraction.

              乘法和处罚优先于加法和减法

•       Operatorsof the same priority are evaluated from left to right.

相同优先级别的运营商从左到右进行评估

•       Parenthesesare used to force prioritized evaluation and to clarify statements.

              用圆括号来强制优先评估，并澄清语句

 

练习2：select last_update_date,unit_price,2 * unit_price + 200 from po_lines_all

 

以上这个语句是吧（）去掉之后，得到的数值是完全不同的，练习1为4400，练习2为4200.