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Maintenance 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.
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The text data type is used to store huge amounts of data. One field can store up 31 to 2GB (2 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. The following command creates a table with three fields using different character string data types:
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Create table Contacts(ContactId char(8), Name varchar(50), Note text)
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Character constants are delimited from the rest of the Transact-SQL code with quotes. For example, the following statement inserts contact information:
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insert into Contacts (ContactId, Name, Note) values ('CO-92-81', 'Tom Jones', 'Tom@trigon.com')
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Microsoft SQL Server 2000 has three character data types for storing Unicode data using non-ASCII character sets. They are equivalent to the char, varchar, and text data types and are called nchar nvarchar ntext
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C h a p t e r 4 : B a s i c Tr a n s a c t - S Q L P r o g r a m m i n g C o n s t r u c t s
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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 (4000 for nchar and nvarchar). The following statement creates the same table, but using Unicode data types:
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Create table Contacts_2(ContactId nchar(8), Name nvarchar(50), Note ntext) go
Unicode character constants are also delimited with quotes but are prefixed with N':
insert into Contacts_2 (ContactId, Name, Note) values (N'CO-92-81', N'Tom Jones', N'Tom@trigonblue.com')
This N' prefix might look a little odd, but you will get used to it. Microsoft documentation is full of samples with Unicode constants. It was 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 text
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.
Date and Time Data Types
SQL Server supports two data types for storing date and time: datetime smalldatetime
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The main difference between these two data types is in the amount of space they occupy. datetime occupies 8 bytes and smalldatetime only 4 bytes. The difference between the two types is the precision of the date stored, and the range of dates that can be used. The precision of smalldatetime is one minute, and it covers dates from January 1, 1900, through June 6, 2079, which is usually more than enough. The precision of datetime is 3.33 ms, and it covers dates from January 1, 1753, to December 31, 9999. Date and time constants are written in Transact-SQL with quote delimiters (as are character strings):
update Contacts_2 Set DateOfBirth = '2/21/1965 10:03 AM' where ContactId = 'CO-92-81'
SQL Server supports many different date and time formats. The Convert() function accepts a parameter that controls the format of date and time functions (explained in detail in 5).
If time is not specified in a constant, SQL Server automatically assigns a default value 12:00 A.M. (midnight). You should keep in mind that SQL Server always records time as a part of these data types. Thus, if you want to select all contacts born on a particular day, you should not use something like this:
select * from Contacts_2 where DateOfBirth = '2/21/1965'
This statement would extract records with DateOfBirth set to midnight of that day. Such a solution might be acceptable if all other applications recording values in the field also make the same mistake. A proper solution would be
select * from Contacts_2 where DateOfBirth >= '2/21/1965' and DateOfBirth < '2/22/1965'
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