Define Tables and Fields

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Define Tables and Fields

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2,2 Xác định cơ sở dữ liệu Bàn Fields A SQL Server không làm nhiều tốt trừ khi bạn có bảng trong nó. Hướng dẫn này không chỉ cung cấp cho bạn thông tin về cách để tạo bảng và các lĩnh vực, nhưng cũng dạy bạn làm thế nào để tên họ và những gì để tìm ra cho khi tạo ra chúng

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  1. 2.2 Define Tables and Fields A SQL Server database doesn't do much good unless you have tables in it. This tutorial not only gives you information on how to create tables and fields, but also teaches you how to name them and what to look out for when creating them. Databases by themselves don't do much for you, but you need to be able to store data in them at the very least. To do this, you need to be able to create tables, which are made up of fields. How do you go about defining tables and fields in VS .NET? Technique Using the Server Explorer and accessing the Table designer, you are going to create a table called tblCustomers in the database that you created in the previous How-To. You will then add columns to the database that will store various pieces of information having to do with individual customers. Last, you will create another table called tblPhones and also create the necessary columns that will contain information about phone information for your customers. Steps After Visual Studio .NET is opened, expand the Server Explorer and locate your new database called Chapter2, created in How-To 2.1. Click on the plus sign by the database names so that the database object categories are listed. 1. Right-click on the Tables node under the Chapter2 database and choose New Table. You will then be presented with the Table Designer. Now you are ready to add the columns. 2. To add a column, you need to set four immediate properties: o Column Name. This is the name of the column. Don't use spaces or special characters, but do use proper case, and make sure that the name you give the column makes sense for what it contains. For instance, if the column is for the last name of a customer, put LastName for the Name property of the column. o Data Type. Depending on the type of data that will be entered into the column, this will be one of the many valid data types for SQL Server, shown here in Table 2.1. Table 2.1. SQL Server Data Types Data Type Description bigint Integer (whole number) data from -263 (-
  2. 9,223,372,036,854,775,808) through 263\up6 - 1\up6 (9,223,372,036,854,775,807). int Integer (whole number) data from -2^31 (- 2,147,483,648) through 231 - 1 (2,147,483,647). smallint Integer data from 215 (-32,768) through 215 - 1 (32,767). tinyint Integer data from 0 through 255. bit Integer data with either a 1 or 0 value. decimal Fixed precision and scale numeric data from -1038 + 1 through 1038 - 1. numeric Functionally equivalent to decimal. money Monetary data values from -263 (- 922,337,203,685,477.5808) through 263 - 1 (+922,337,203,685,477.5807), with accuracy to a ten- thousandth of a monetary unit. smallmoney Monetary data values from -214,748.3648 through +214,748.3647, with accuracy to a ten-thousandth of a monetary unit. float Floating precision number data from -1.79E + 308 through 1.79E + 308. real Floating precision number data from -3.40E + 38 through 3.40E + 38. datetime Date and time data from January 1, 1753, through December 31, 9999, with an accuracy of three- hundredths of a second, or 3.33 milliseconds. smalldatetime Date and time data from January 1, 1900, through June 6, 2079, with an accuracy of one minute. char Fixed-length non-Unicode character data with a maximum length of 8,000 characters. varchar Variable-length non-Unicode data with a maximum length of 8,000 characters. text Variable-length non-Unicode data with a maximum length of 231 - 1 (2,147,483,647) characters.
  3. nchar Fixed-length Unicode data with a maximum length of 4,000 characters. nvarchar Variable-length Unicode data with a maximum length of 4,000 characters. ntext Variable-length Unicode data with a maximum length of 230 - 1 (1,073,741,823) characters. binary Fixed-length binary data with a maximum length of 8,000 bytes. Varbinary Variable-length binary data with a maximum length of 8,000 bytes. image Variable-length binary data with a maximum length of 231 - 1 (2,147,483,647) bytes. cursor A reference to a cursor. sql_variant A data type that stores values of various SQL Server- supported data types, except text, ntext, timestamp, and sql_variant. table A special data type that stores a result set for later processing. timestamp A database-wide unique number that is updated every time a row is updated. uniqueidentifier A globally unique identifier (GUID). o Tip Unicode is a character-encoding standard that uses 16-bit code values. This standard is used worldwide to represent all the characters that are used in modern computing. Traditional character sets are the previous character- encoding standards-such as the Windows ANSI character set-that use 8-bit code values or combinations of 8-bit values to represent the characters used in a specific language or geographical region. It is recommended that you use Unicode data types-nchar, nvarchar, and ntext-rather than their non-Unicode
  4. counterparts. Also, use the variant length type data types whenever possible. Doing so will save disk space and save you from having to trim your values when you want to display the data in the fields. o Length. This varies depending on which data type you choose. For text data types, this will be the maximum length you expect to be entered into the column. o Allow Nulls. This specifies whether you will allow null values to be saved in the record for the column. This means that the user doesn't have to enter a value at all. This is sometimes a bad idea, such as when you have specific data that has to be entered, like Social Security Numbers. For the first few columns in the table, enter the following data. You can see how the table will look in Figure 2.3. Figure 2.3. Fields for your first table. Column Name Data Type Length Allow Nulls
  5. LastName nvarchar 50 Unchecked FirstName nvarchar 50 Unchecked Address nvarchar 50 Checked City nvarchar 50 Checked State nvarchar 2 Checked ZipCode nvarchar 9 Checked BirthDate datetime 8 Checked MailingList bit 1 Checked EstimatedSales money 8 Checked 3. Save the table by clicking on the X in the top-right corner of the Table Designer, and name the table tblCustomers when prompted. After you click the X, you are asked if you want to save the table. Then you are prompted for the name to save the table as. How It Works By entering the information in the various properties for each column, you are specifying how you want the data in your database to be treated. Generally, your users won't create tables; you will create the tables for the users, and they will fill the data into the tables using your applications. When the user fills in the data, the application and SQL Server control what type of data goes into the table, starting with what the data type of the data is and what the allowed length is. The Allow Nulls property determines whether the user even has to enter data. Comments Creating the tables, made up of columns and rows, is the basis for the database's purpose: storing information. Making sure that you use logical, descriptive names for columns, along with data types that help control how the data gets entered into the database, is key to a successful database. Plan out your tables ahead of time, examining each real-world object, and transfer those properties to the columns that make up the tables that represent your objects.
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