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SQL Data Types

In SQL databases, data types play an important role in defining and managing information. This guide explores the purpose and usage of SQL data types, covering numeric, date/time, character/string, and binary types. We also examine their variations across popular databases like MySQL, SQL Server, and Oracle. By the end, you'll have the knowledge to make informed decisions when designing databases and optimizing SQL queries. Let's uncover the power of SQL data types and elevate your database management skills.


SQL Data Type is an attribute that specifies the type of data of any object. Each column, variable, and expression has a related data type in SQL. SQL data type is a type of data in a table column or variable. It is a mandatory and essential step in designing a table. A table with inappropriate data types can lead to various issues such as improper query optimization, performance issues, and data truncation.


What is Data Type and why it is important in SQL?

In SQL, a data type is a classification of data based on its characteristics and the operations that can be performed on it. It specifies the type of data that can be stored in a column or variable in a database table. Data types define the format, size, and range of values that can be stored, as well as the operations that can be performed on those values.


Data types are important in SQL for several reasons:

  1. Data Integrity: By specifying the appropriate data type for a column, you ensure that only valid data is stored in that column. For example, if a column is defined as an integer data type, it can only store whole numbers, preventing the insertion of invalid or incompatible data.

  2. Storage Optimization: Different data types have different storage requirements. By choosing the appropriate data type, you can optimize storage and improve performance. For example, using a smaller data type like "SMALLINT" instead of "INT" can save storage space if the column's range of values falls within the smaller data type's limits.

  3. Data Validation: Data types allow for data validation and constraint enforcement. For example, you can define a column as a "DATE" type to ensure that only valid dates are stored in that column. SQL will enforce the data type's constraints, preventing the insertion of incorrect data.

  4. Query Optimization: SQL query execution can be optimized based on the data types used in the query. The database engine can choose the most efficient execution plan based on the data types involved. For example, if a column is indexed and the query specifies a data type-compatible search condition, the index can be utilized efficiently to speed up the query.

  5. Data Manipulation: Data types define the operations that can be performed on the data. SQL provides a wide range of built-in functions and operators that work specifically with certain data types. For example, you can perform mathematical calculations on numeric data types or use string functions on character data types.


Differences and Similarities between Data Types in Different SQL Databases: MySQL, SQL Server, and MS Access

Here is a general overview of the differences and similarities between numeric, date/time, character/string, and binary data types in these databases:


Numeric Data Types:

  • Similarities: All three databases support common numeric data types such as INTEGER, DECIMAL/NUMERIC, FLOAT, and DOUBLE.

  • Differences:

    • SQL Server has additional numeric data types like MONEY and SMALLMONEY for handling currency values.

    • MySQL has specific data types like TINYINT, SMALLINT, MEDIUMINT, and BIGINT for integers with varying storage sizes.

    • MS Access uses LONG for large integers and CURRENCY for currency values.

Date/Time Data Types:

  • Similarities: All three databases support standard date/time data types such as DATE, TIME, and TIMESTAMP.

  • Differences:

    • SQL Server uses DATETIME2 with varying precision for a higher range of dates and times.

    • MS Access uses DATETIME and stores date and time information together.

    • MySQL has additional date/time data types like DATETIME, TIMESTAMP, and YEAR.

Character/String Data Types:

  • Similarities: All three databases support common character/string data types like CHAR, VARCHAR, and TEXT.

  • Differences:

    • SQL Server uses NVARCHAR and NCHAR for storing Unicode character data.

    • MS Access uses MEMO for storing large text and VARCHAR for variable-length character data.

    • MySQL has additional data types like TINYTEXT, MEDIUMTEXT, and LONGTEXT for storing larger amounts of text.

Binary Data Types:

  • Similarities: All three databases support binary data types such as BINARY and VARBINARY.

  • Differences:

    • SQL Server has IMAGE data type for storing binary large objects (BLOBs).

    • MS Access uses the OLE Object data type for storing binary objects like images and documents.

    • MySQL has additional data types like BLOB, TINYBLOB, MEDIUMBLOB, and LONGBLOB for storing binary large objects.

Important: Specific data types and their behaviors may vary further based on the version and configuration of each database. It's recommended to refer to the official documentation of each database for the most accurate and up-to-date information on data types.


SQL Data Types

SQL Data Types

1. Numeric

Numeric data types are used to store numbers in database columns. These types can be categorized into exact and approximate numeric types, based on the need to preserve precision and scale.

  1. Exact numeric types, such as INTEGER, BIGINT, DECIMAL, NUMERIC, NUMBER, and MONEY, are used when the precision and scale of the values need to be preserved. These types store numbers without any loss of precision.

  2. Approximate numeric types, including DOUBLE PRECISION, FLOAT, and REAL, are used when the precision needs to be preserved, but the scale can be floating. These types provide an approximation of the numeric values and allow for more flexibility in terms of scale.

Below is the list of the Numeric Data Types:


1. BIT: Represents a bit-value type with a specified size. The size can range from 1 to 64, with a default of 1.


2. INTEGER (INT): Stores whole numbers with an implied scale of zero. It can hold values between -2^31 and 2^31-1, and assigning values outside this range will result in an error.


3. SMALLINT: Stores whole numbers with an implied scale of zero but has a smaller range than INTEGER. It can hold values between -2^15 and 2^15-1.


4. TINYINT: It is a very small integer data type. It can store signed values in the range of -128 to 127 and unsigned values from 0 to 255. The size parameter specifies the maximum display width, which is typically set to 255.


5. DECIMAL: Used for precise representation of numbers. It accepts numeric values with a defined precision (total number of digits) and scale (number of digits after the decimal point). The scale cannot exceed the precision.


6. NUMERIC: It is similar to DECIMAL and is treated the same way in many databases.


7. FLOAT: Used for the representation of floating-point values. It can store numbers with specified precision, up to a maximum of 64. If no precision is specified, the default is 64.


8. REAL: Used to store floating-point values. It has a precision of 64 and does not require any additional parameters during declaration.


2. Date/Time

The date/time data type in SQL is used to store values that contain both date and time information. It combines the date and time representations, typically based on a 24-hour clock with fractional seconds.


Here we have different date/time data types and their usage:


1. DATE: Accepts date values in the format 'YYYY-MM-DD'. It does not require any additional parameters. The month should be between 1 and 12, the day should be between 1 and 31 (depending on the month), and the year should be between 0 and 9999. Example: DATE '1999-04-04'.


2. TIME: Accepts time values in the format 'HH:MM:SS'. It can also include an optional fractional value to represent nanoseconds. The hour should be between 0 and 23, the minute should be between 00 and 59, and the second should be between 00 and 61.999999. Example: TIME '07:30:00'.


3. DATETIME: Represents a combination of date and time. Its format is 'YYYY-MM-DD hh:mm:ss', and it supports a range from '1000-01-01 00:00:00' to '9999-12-31 23:59:59'.


4. TIMESTAMP: Represents a combination of date and time. It follows the format 'YYYY-MM-DD HH:MM:SS', where there is a space separating the date and time portions. The same specifications and restrictions for the DATE and TIME data types apply to TIMESTAMP. Example: TIMESTAMP '1999-04-04 07:30:00'.


5. INTERVAL: Used to represent a period of time, such as years, months, days, hours, minutes, or seconds. It allows performing calculations and manipulations on date/time values.


3. Binary

In SQL Server, the binary string data types are Binary, VARBINARY, and VARBINARY(MAX). These data types are used to store raw binary data.


Let's explore each of these data types:


1. BINARY: It is similar to the CHAR data type but stores binary byte strings. The size parameter specifies the column length in bytes, with the default size being 1.


2. VARBINARY: It is similar to the VARCHAR data type but stores binary byte strings. The size parameter specifies the maximum column length in bytes.


3. VARBINARY(MAX): It is an extension of VARBINARY and is used to store binary data with a length of up to (32,767 - 1) bytes. It allows storing larger binary data than the maximum size allowed for VARBINARY.


4. YEAR: Used to represent a year in a four-digit format. It allows specifying years from 1901 to 2155 and the special value 0000. The year can also be stored in a two-digit format, representing the years from 70 to 69, which corresponds to the years 1970 to 2069.


4. Character/String

A string data type is a sequence of characters, either as a literal constant or as some kind of variable. The latter may allow its elements to be mutated and the length changed, or it may be fixed (after creation).


Let's explore two commonly used string data types:


1. CHAR: Used to store fixed-length character strings, including Unicode. When declaring a CHAR column, you need to specify the desired length of the character string using the syntax CHAR(n), where n represents the length. If no length is specified, the default length is 1.


The length of a CHAR column can range from 1 to the maximum page size of the table. If you attempt to store a string longer than the defined length, it will be truncated. If the truncated string contains non-blank characters, an error will be raised. If you need to store variable-length strings, consider using the VARCHAR data type.


2. VARCHAR: Used to store variable-length character strings, including Unicode. It allows you to define a maximum length for the character string using the syntax VARCHAR(n), where n represents the maximum length.


Unlike CHAR, VARCHAR columns can store strings of varying lengths, up to the maximum length specified in the declaration. If you assign a string that exceeds the defined maximum length, it will be truncated to fit. If any truncated characters are not blank, an error will be raised.


Important: The maximum length for VARCHAR is typically determined by the current table page size. Additionally, if you need to store very large character strings, you might consider using a Character Large Object (CLOB) data type.


Miscellaneous Data Types


1. CLOB: The Character Large Object (CLOB) data type allows storing character strings longer than what is permitted in CHARACTER or VARCHAR data types.


The CLOB declaration uses the following syntax to specify the length of the CLOB in bytes:

n [K | M | G]


In the above syntax, n is an unsigned integer that represents the length. K, M, and G correspond to Kilobytes, Megabytes or Gigabytes, respectively. If K, M, or G is specified in addition to n, then the actual length of n is the following:

  • K = n * 1024

  • M = n * 1,048,576

  • G = n * 1,073,741,824

The maximum size allowed for CLOB data types is 2 gigabytes. If no length is specified, the default length is one byte. CLOB values can range in length from one byte up to the specified length.


2. BLOB: The Binary Large Object (BLOB) data type is used for storing binary values. Similar to CLOB, the length of a BLOB is specified using the syntax "n [K | M | G]".


The maximum size allowed for BLOB data types is also 2 gigabytes. If no length is specified, the default length is one byte. BLOB values can vary in length from one byte up to the specified length.


3. XML (or JSON): The XML data type is a built-in data type in SQL Server that allows storing XML data. It is similar to other built-in types like int and varchar. You can use the XML data type as a column type when creating a table, as a variable type, a parameter type, or in CAST and CONVERT operations. SQL Server also provides support for storing JSON data, allowing you to work with JSON documents directly within the database.


4. GEOMETRY (or GEOGRAPHY): The GEOMETRY data type is used to store geometric data, such as points, lines, and polygons. It allows you to perform spatial calculations and queries on the stored data. The GEOGRAPHY data type is similar to GEOMETRY but is specifically designed for storing geographic data, such as coordinates on the Earth's surface. Both data types enable advanced spatial analysis and indexing capabilities.


5. TABLE (or CURSOR): The TABLE data type allows you to define a table variable, which can hold multiple rows and columns of data. It is useful for temporarily storing and manipulating data within a query or procedure. Cursors, on the other hand, are used to iterate through the result set of a query, allowing you to perform row-by-row processing. Cursors are often used in more complex scenarios where sequential access to the result set is required.


Unicode Character

A UNICODE character uses multiple bytes to store the data in the database. This means that by using UNICODE it is possible to process characters of various writing systems in one document.


1. VARCHAR: Used to store variable-length character strings, including Unicode characters. The maximum length of the string is specified in the data type declaration using a positive integer within parentheses, such as VARCHAR(n), where n represents the maximum number of characters. The length can range from 1 to the current table page size. If a value longer than the defined maximum length is assigned, the string will be truncated to fit. If any truncated characters are non-blank, an error is raised.


2. NVARCHAR: Designed to store character data in a variable-length field. It supports both single-byte and multibyte characters based on the code set of your database locale. NVARCHAR allows storing text in various writing systems and can accommodate letters, digits, and other characters. The length of an NVARCHAR column is specified when it is created.


3. NVARCHAR(MAX): It is a variable-size string data type that allows for storing large amounts of text. The size is defined using a length specifier, "n," which represents the number of byte-pairs and can range from 1 to 4,000. Additionally, the "MAX" option indicates that the maximum storage size is 2^30-1 characters (2 GB). The actual storage size is two times the number of bytes plus 2 bytes.


4. NTEXT: Used for storing variable-width character strings with a maximum size of up to 2 GB of text data. It is designed for handling large amounts of text in a database, and it supports Unicode characters. The NTEXT data type is no longer recommended for use in newer versions of SQL Server, and the NVARCHAR(MAX) data type is preferred instead.


Using Unicode character data types like NVARCHAR and NTEXT allows processing and storing characters from various writing systems, enabling multi-language support and compatibility.


Best Practices for working with SQL Data Types:


Best Practice 1: Choose the appropriate data type for each column or variable based on the type and range of values it can hold, the storage requirements, and the performance implications. For example, use INT instead of BIGINT if you don’t need to store very large numbers, or use VARCHAR instead of CHAR if you don’t need to store fixed-length strings.


Best Practice 2: Use consistent data types across tables and queries to avoid implicit conversions and potential errors. For example, use the same data type for primary keys and foreign keys, or use the same data type for parameters and arguments in stored procedures and functions.


Best Practice 3: Use explicit conversions when you need to change the data type of a value or expression. For example, use CAST or CONVERT functions to convert a string to a date or a numeric value to a string. Avoid relying on implicit conversions that may vary depending on the database settings or the data type precedence rules.


Best Practice 4: Use data type synonyms for ISO compatibility and portability. For example, use DATE instead of DATETIME or TIME instead of DATETIME2 if you only need to store the date or time component. Data type synonyms are equivalent to the system data types in SQL Server and can be used interchangeably.


Best Practice 5: Use NULL values carefully and consistently. NULL values indicate that a value is unknown or missing, and they can affect the results of calculations, comparisons, and aggregations. For example, use IS NULL or IS NOT NULL operators to check for NULL values, or use COALESCE or ISNULL functions to replace NULL values with default values.

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