CHAPTER 5Getting More Than Simple Columns
Intro Value expression,itcontains column names, literal values, functions, orother value
expressions
The type of data
TheSQL Standard defines seven general categories of types of data—character,
nationalcharacter, binary, numeric, Boolean, datetime, and interval.
The national character data type is the same as thecharacter data type except that it drawsits charactersfrom ISO-definedforeign language
character sets.
BOOLEAN数据类型可以使用TINYINT存储.
CAST
Changing Data Types
data_type需要查看具体数据库实现文档,比如MySQL
The type for
the result can be one of the following values:
- BINARY[(N)]
- CHAR[(N)]
源文档 <http://dev.mysql.com/doc/refman/5.0/en/cast-functions.html#function_cast>
比如
SELECTOrderNumber, OrderDate, ShipDate,
CAST(ShipDate– OrderDate AS INTEGER)
ASDaysElapsed
FROMOrders
在MySQL中需要转为
SELECTOrderNumber, OrderDate, ShipDate,
CAST(ShipDate- OrderDate AS DECIMAL)
ASDaysElapsed
FROMOrders
Literal Value
分为字符串常量值,数值常量值和日期常量值.
如果使用单引号包含字符串,字符串中包含单引号,字符串中的单引号请使用两次以示与引用的单引号区别开来.
SQL‘The Vendor"s name is: ‘
Displayed as The Vendor‘s name is:
Types of Expressions
CONCATENATION
SQL字符串串接
? Note Of the major database systems,we found that onlyIBM’s DB2 and
Informixand Oracle’s Oracle support the SQL Standard operator for concatenation.
MicrosoftOffice Access supports & and + as concatenation
operators,Microsoft SQL Server and Ingres support +, and in MySQL you
mustuse the CONCAT function. In all the examples in the book,we use the
SQLStandard || operator. In the sample databases on the CD,we use the
appropriateoperator for each database type (Microsoft Access, Microsoft
SQLServer, and MySQL).
由上图可知SQL标准字符串串接用||，但是在MySQL使用会被当作逻辑运算符。MySQL使用concat内建函数串接字符串。而有的数据库使用直观的+串接字符串.
DATE AND TIME ARITHMETIC
MySQL需要将日期常量值转为相应类型再做计算,另外需要使用相应的日期函数计算
CAST(‘2016-11-22‘AS DATE)
CAST(‘03:30:25‘AS TIME)
CAST(‘2008-09-2914:25:00‘ AS DATETIME)
比如
SELECTStfLastName || ‘, ‘ || StfFirstName AS Staff,
DateHired,
CAST(CAST(‘2007-10-01‘- DateHired AS INTEGER)
/365 AS INTEGER)
ASYearsWithSchool
FROMStaff
ORDERBY StfLastName, StfFirstName
在MySQL中转为
SELECTCONCAT(StfLastName , ‘, ‘ , StfFirstName) AS Staff,
DateHired,
CAST(DATEDIFF(CAST(‘1990-01-11‘AS date) , DateHired )/365 as decimal)
ASYearsWithSchool
FROMStaff
ORDERBY StfLastName, StfFirstName
That"Nothing" Value:Null
判断Value Expression是否为NULL的时候请不要使用ValueExpression
= NULL,这是常犯的小错误.