All the data values of a column must be of the same data type. (The only exception specifies the values of the SQL_VARIANT data type.) Transact-SQL uses different data types, which can be categorized as follows:
- Numeric data types
- Character data types
- Temporal (date and/or time) data types
- Miscellaneous data types
The following sections describe all these categories.
Numeric Data Types
Numeric data types are used to represent numbers. The following table shows the list of all numeric data types:
|Represents integer values that can be stored in 4 bytes. The range of values is –2,147,483,648 to 2,147,483,647. INT is the short form for INTEGER.
|Represents integer values that can be stored in 2 bytes. The range of values is –32768 to 32767.
|Represents nonnegative integer values that can be stored in 1 byte. The range of values is 0 to 255.
|Represents integer values that can be stored in 8 bytes. The range of values is –263 to 263 – 1.
|Describes fixed-point values. The argument p (precision) specifies the total number of digits with assumed decimal point s (scale) digits from the right. DECIMAL values are stored, depending on the value of p, in 5 to 17 bytes. DEC is the short form for DECIMAL.
|Synonym for DECIMAL.
|Used for floating-point values. The range of positive values is approximately 2.23E – 308 to 1.79E + 308, and the range of negative values is approximately –1.18E – 38 to –1.18E + 38 (the value zero can also be stored).
|Represents floating point values, like REAL. p defines the precision, with p < 25 as single precision (stored in 4 bytes) and p >= 25 as double precision (stored in 8 bytes).
|Used for representing monetary values. MONEY values correspond to 8-byte DECIMAL values and are rounded to four digits after the decimal point.
|Corresponds to the data type MONEY but is stored in 4 bytes.
Character Data Types
There are two general forms of character data types. They can be strings of single-byte characters or strings of Unicode characters. (Unicode uses several bytes to specify one character.) Further, strings can have fixed or variable length. The following character data types are used:
|Represents a fixed-length string of single-byte characters, where n is the number of characters inside the string. The maximum value of n is 8000. CHARACTER(n) is an additional equivalent form for CHAR(n). If n is omitted, the length of the string is assumed to be 1.
|Describes a variable-length string of single-byte characters (0 < n ≤ 8000). In contrast to the CHAR data type, the values for the VARCHAR data type are stored in their actual length. This data type has two synonyms: CHAR VARYING and CHARACTER VARYING.
|Stores fixed-length strings of Unicode characters. The main difference between the CHAR and NCHAR data types is that each character of the NCHAR data type is stored in 2 bytes, while each character of the CHAR data type uses 1 byte of storage space. Therefore, the maximum number of characters in a column of the NCHAR data type is 4000.
|Stores variable-length strings of Unicode characters. The main difference between the VARCHAR and the NVARCHAR data types is that each NVARCHAR character is stored in 2 bytes, while each VARCHAR character uses 1 byte of storage space. The maximum number of characters in a column of the NVARCHAR data type is 4000.
Temporal Data Types
Transact-SQL supports the following temporal data types:
The DATETIME and SMALLDATETIME data types specify a date and time, with each value being stored as an integer value in 4 bytes or 2 bytes, respectively. Values of DATETIME and SMALLDATETIME are stored internally as two separate numeric values. The date value of DATETIME is stored in the range 01/01/1753 to 12/31/9999. The analog value of SMALLDATETIME is stored in the range 01/01/1900 to 06/06/2079. The time component is stored in the second 4-byte (or 2-byte for SMALLDATETIME) field as the number of three-hundredths of a second (DATETIME) or minutes (SMALLDATETIME) that have passed since midnight.
The use of DATETIME and SMALLDATETIME is rather inconvenient if you want to store only the date part or time part. For this reason, SQL Server introduced the data types DATE and TIME, which store just the DATE or TIME component of a DATETIME, respectively. The DATE data type is stored in 3 bytes and has the range 01/01/0001 to 12/31/9999. The TIME data type is stored in 3–5 bytes and has an accuracy of 100 nanoseconds (ns).
The DATETIME2 data type stores high-precision date and time data. The data type can be defined for variable lengths depending on the requirement. (The storage size is 6–8 bytes). The accuracy of the time part is 100 ns. This data type isn’t aware of Daylight Saving Time.
All the temporal data types described thus far don’t have support for the time zone. The data type called DATETIMEOFFSET has the time zone offset portion. For this reason, it is stored in 6–8 bytes. (All other properties of this data type are analogous to the corresponding properties of DATETIME2.)
The date value in Transact-SQL is by default specified as a string in a format like ‘mmm dd yyyy’ (e.g., ‘Jan 10 1993’) inside two single quotation marks or double quotation marks. (Note that the relative order of month, day, and year can be controlled by the SET DATEFORMAT statement. Additionally, the system recognizes numeric month values with delimiters of / or –.) Similarly, the time value is specified in the format ‘hh:mm’ and the Database Engine uses 24-hour time (23:24, for instance).
Examples 4.4 and 4.5 show the different ways date and time values can be written using the different formats.
The following date descriptions can be used:
'28/5/1959' (with SET DATEFORMAT dmy)
'May 28, 1959'
The following time expressions can be used:
Binary and BIT Data Types
BINARY and VARBINARY are the two binary data types. They describe data objects being represented in the internal format of the system. They are used to store bit strings. For this reason, the values are entered using hexadecimal numbers.
The values of the BIT data type are stored in a single bit. Therefore, up to 8 bit columns are stored in 1 byte. The following table summarizes the properties of these data types:
|Specifies a bit string of fixed length with exactly n bytes (0 < n ≤ 8000).
|Specifies a bit string of variable length with up to n bytes (0 < n ≤ 8000).
|Used for specifying the Boolean data type with three possible values: FALSE, TRUE, and NULL.
Large Object Data Types
Large objects (LOBs) are data objects with the maximum length of 2GB. These objects are generally used to store large text data and to load modules and audio/video files. Transact-SQL supports the following LOB data types:
Starting with SQL Server 2005, you can use the same programming model to access values of standard data types and LOBs. In other words, you can use convenient system functions and string operators to work with LOBs.
The Database Engine uses the max parameter with the data types VARCHAR, NVARCHAR, and VARBINARY to define variable-length columns. When you use max by default (instead of an explicit value), the system analyzes the length of the particular string and decides whether to store the string as a convenient value or as a LOB. The max parameter indicates that the size of column values can reach the maximum LOB size of the current system.
Although the database system decides how a LOB will be stored, you can override this default specification using the sp_tableoption system procedure with the LARGE_VALUE_TYPES_OUT_OF_ROW option. If the option’s value is set to 1, the data in columns declared using the max parameter will be stored separately from all other data. If this option is set to 0, the Database Engine stores all values for the row size < 8060 bytes as regular row data.
Since SQL Server 2008, you can apply the new FILESTREAM attribute to a VARBINARY(max) column to store large binary data directly in the NTFS file system. The main advantage of this attribute is that the size of the corresponding LOB is limited only by the file system volume size. (This storage attribute will be described in the upcoming “Storage Options” section.)
UNIQUEIDENTIFIER Data Type
As its name implies, a value of the UNIQUEIDENTIFIER data type is a unique identification number stored as a 16-byte binary string. This data type is closely related to the globally unique identifier (GUID), which guarantees uniqueness worldwide. Hence, using this data type, you can uniquely identify data and objects in distributed systems.
The initialization of a column or a variable of the UNIQUEIDENTIFIER type can be provided using the functions NEWID and NEWSEQUENTIALID, as well as with a string constant written in a special form using hexadecimal digits and hyphens. (The functions NEWID and NEWSEQUENTIALID are described in the section “System Functions” later in this chapter.)
A column of the UNIQUEIDENTIFIER data type can be referenced using the keyword ROWGUIDCOL in a query to specify that the column contains ID values. (This keyword does not generate any values.) A table can have several columns of the UNIQUEIDENTIFIER type, but only one of them can have the ROWGUIDCOL keyword.
SQL_VARIANT Data Type
The SQL_VARIANT data type can be used to store values of various data types at the same time, such as numeric values, strings, and date values. (The only types of values that cannot be stored are TIMESTAMP values.) Each value of an SQL_VARIANT column has two parts: the data value and the information that describes the value. (This information contains all properties of the actual data type of the value, such as length, scale, and precision.)
Transact-SQL supports the SQL_VARIANT_PROPERTY function, which displays the attached information for each value of an SQL_VARIANT column. For the use of the SQL_VARIANT data type, see Example 5.5 in Chapter 5.
HIERARCHYID Data Type
The HIERARCHYID data type is used to store an entire hierarchy. (For instance, you can use this data type to store a hierarchy of all employees or a hierarchy of all folder lists.) It is implemented as a Common Language Runtime (CLR) user-defined type that comprises several system functions for creating and operating on hierarchy nodes. The following functions, among others, belong to the methods of this data type: GetLevel(), GetAncestor(), GetDescendant(), Read(), and Write().
TIMESTAMP Data Type
The TIMESTAMP data type specifies a column being defined as VARBINARY(8) or BINARY(8), depending on nullability of the column. The system maintains a current value (not a date or time) for each database, which it increments whenever any row with a TIMESTAMP column is inserted or updated and sets the TIMESTAMP column to that value. Thus, TIMESTAMP columns can be used to determine the relative time at which rows were last changed. (ROWVERSION is a synonym for TIMESTAMP.)