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Character string data types

07 Feb 2007 | McGraw-Hill

Expert advice on database development

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Data Types

Data types specify the type of information (such as number, string, picture, date) that can be stored in a column or a variable.

SQL Server recognizes 28 system-defined data types. Apart from these data types, you can create user-defined data types in T-SQL

and in .NET to fulfill specific needs.

The following are the categories of system-defined data types:

Character strings

Unicode character strings

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Figure 3-1 Grouping objects using a schema in the Object Browser.

Date and time

Approximate numeric

Exact numeric

Integer numbers

Monetary

Binary

Special

NOTE

In some cases, you can use different identifiers to refer to a data type in T-SQL code. For example, the char data type can be

referenced as character, and varchar can be referenced as character varying. Some of these synonyms are based on ANSI SQL-92

standard requirements.

Figure 3-1 Grouping objects using a schema in the Object Browser

Character Strings

Character data types store character strings. The four different character types vary in length and storage characteristics:

char

varchar

varchar(max)

text

The char data type is used to store strings of fixed size. As noted earlier, the maximum size of this data type is 8,000 characters,

which is a significant increase over the 255-character limit in early versions. When a variable or a table column is assigned with a

string that is shorter than its nominal size, it is padded with trailing spaces to fill the specified field length.

The varchar data type stores strings of variable size up to 8,000 characters long. When a character value whose length is less than

the nominal size is assigned to the column or variable, SQL Server does not add trailing spaces to it, but records it as is. varchar

data types occupy two additional bytes in order to record the length of the string.

NOTEMaintenance of this information requires some additional computation during I/O operation, but that time is usually countered by

savings in the space required. A record using such columns occupies less space, and more records fit into a single page. Therefore,

SQL Server reads more records when accessing data, and it is more likely that a single page contains the information that the user is

looking for.

The text data type is used to store huge amounts of data. One field can store up to 2GB (231 1 bytes) of information. Only a

16-byte pointer to this data is stored in the table. Therefore, additional processing overhead is involved with the use of text columns.

There are special functions for processing text values.

NOTE

You should avoid using this data type because Microsoft plans to remove it in future versions of SQL Server. You should use the

new varchar(max) data type instead.

Varchar(max) looks and operates like the varchar data type, but it is by internal structure and functionality actually much more like


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the text data type.

It is also designed to store large strings (up to 2GB). The string is stored in special data pages, not in the data page with the rest of

the record. Its biggest advantage is that most string operating functions that work with varchar also work with varchar(max); however,

there are a few new functions for additional functionality. The following command creates a table with four fields using different

character string data types:

Create table Contacts(ContactId char(8),

Name varchar(50),

Note text,

Resume varchar(max))

Character constants are delimited from the rest of the Transact-SQL code with quotes. For example, the following statement inserts

contact information:

insert into Contacts (ContactId, Name, Note, Resume)

values ('CO-92-81', 'Tom Jones', '[email protected]', 'N/a')

Unicode Character Strings

Microsoft SQL Server 2005 has four character data types for storing Unicode datausing non-ASCII character sets. They are

equivalent to the char, varchar, varchar(max), and text data types and are called

nchar

nvarchar

nvarchar(max)

ntext

The main difference between these new data types and the older character data types is that the new data types can hold Unicode

characters, which occupy 2 bytes per character. Therefore, the maximum string length that they can store is half that of the

corresponding older data types (4,000 for nchar and nvarchar).

The following statement creates the same table as the previous example but uses Unicode data types:

Create table Contacts_2(ContactId nchar(8),

Name nvarchar(50),

Note ntext,

Resume nvarchar(max))

go

Unicode character constants are also delimited with quotes but are prefixed with N':

insert into Contacts_2 (ContactId, Name, Note, Resume)

values (N'CO-92-81', N'Tom Jones', N'[email protected]', N'N/a')

This N' prefix might look a little odd, but you will get used to it. Microsoft documentation is full of samples with Unicode constants. Itwas some time before I discovered the reason Microsoft uses N' as a prefix. It stands for "National." In fact, acceptable alternative

identifiers for these data types are

National char

National char varying

National char varying(max)

National text

TIP

Typically, it is not a problem if you omit the N' prefix on constants. SQL Server automatically converts the string to its Unicode

equivalent. Naturally, it is better to insert it whenever you are dealing with Unicode columns or variables, but it is not a big problem.

The CPU will just have to perform a couple of extra cycles to make the conversion.

However, there are cases in which it becomes a problem. When your string constant is part of a query criterion, then the presence of

the N' prefix might significantly affect execution of the query. If the column is defined as a non-Unicode string and the criterion is

specified with the N' prefix, SQL Server converts every row of the table to compare it with the Unicode constant. As a result, the

query performs a table scan instead of using an index.

Basic Transact-SQL Programming Constructs

Home: Introduction

Tip 1: T-SQL identifiers

Tip 2: Database object qualifiers

Tip3: Character string data types

Tip 4: Date, time and number data types

Tip 5: Special data types-Part 1

Tip 6: Special data types-Part 2

Tip 7: Local variables

Tip 8: Global variables


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Tip 9: Table variables

Tip 10: Flow control statements

Tip 11: Blocks, looping and conditional statements

Tip 12: Unconditional and scheduled execution

Tip 13: Transact SQL cursors

Tip 14: Cursor related statements

Tip 15: Problems and justified use of cursors

The previous tip is from "15 tips in 15 minutes: Basic Transact-SQL Programming Constructs,"

excerpted from Chapter 3, of the book"Microsoft SQL Server 2005 Stored Procedure Programming in

T-SQL & .NET"by Dejan Sunderic, courtesy of McGraw-Hill Publishing.

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