Properly defining the fields in a table is important to the overall optimization of your database. You should use only the type and size of field you really need to use; don't define a field as 10 characters wide if you know you're only going to use 2 charactersthat's 8 extra characters the database has to account for, even if they're unused. These field types are also referred to as data types, as in the "type of data" you will be storing in those fields.
MySQL uses many different data types, broken into three categories: numeric, date and time, and string types. Pay close attention because defining the data type is more important than any other part of the table creation process.
Numeric Data Types
MySQL uses all the standard ANSI SQL numeric data types, so if you're coming to MySQL from a different database system, these definitions will look familiar to you. The following list shows the common numeric data types and their descriptions.
By the Way
The terms signed and unsigned will be used in the list of numeric data types. If you remember your basic algebra, you'll recall that a signed integer can be a positive or negative integer, whereas an unsigned integer is always a non-negative integer.
INT A normal-sized integer that can be signed or unsigned. If signed, the allowable range is from 2147483648 to 2147483647. If unsigned, the allowable range is from 0 to 4294967295. You can specify a width of up to 11 digits.
TINYINT A very small integer that can be signed or unsigned. If signed, the allowable range is from 128 to 127. If unsigned, the allowable range is from 0 to 255. You can specify a width of up to 4 digits.
SMALLINT A small integer that can be signed or unsigned. If signed, the allowable range is from 32768 to 32767. If unsigned, the allowable range is from 0 to 65535. You can specify a width of up to 5 digits.
MEDIUMINT A medium-sized integer that can be signed or unsigned. If signed, the allowable range is from 8388608 to 8388607. If unsigned, the allowable range is from 0 to 16777215. You can specify a width of up to 9 digits.
BIGINT A large integer that can be signed or unsigned. If signed, the allowable range is from 9223372036854775808 to 9223372036854775807. If unsigned, the allowable range is from 0 to 18446744073709551615. You can specify a width of up to 11 digits.
FLOAT(M,D) A floating-point number that cannot be unsigned. You can define the display length (M) and the number of decimals (D). This is not required and will default to 10,2, where 2 is the number of decimals and 10 is the total number of digits (including decimals). Decimal precision can go to 24 places for a FLOAT.
DOUBLE(M,D) A double precision floating-point number that cannot be unsigned. You can define the display length (M) and the number of decimals (D). This is not required and will default to 16,4, where 4 is the number of decimals. Decimal precision can go to 53 places for a DOUBLE. REAL is a synonym for DOUBLE.
DECIMAL(M,D) An unpacked floating-point number that cannot be unsigned. In unpacked decimals, each decimal corresponds to one byte. Defining the display length (M) and the number of decimals (D) is required. NUMERIC is a synonym for DECIMAL.
Of all the MySQL numeric data types, you will likely use INT most often. You can run into problems if you define your fields to be smaller than you actually need; for example, if you define an ID field as an unsigned TINYINT, you won't be able to successfully insert that 256th record if ID is a primary key (and thus required).
Date and Time Types
MySQL has several data types available for storing dates and times, and these data types are flexible in their input. In other words, you can enter dates that are not really days, such as February 30February has only 28 or 29 days, never 30. Also, you can store dates with missing information. For example, if you know that someone was born sometime in November of 1980, you can use 1980-11-00, where "00" would have been for the day, if you knew it.
The flexibility of MySQL's date and time types also means that the responsibility for date checking falls on the application developer (that would be you). MySQL checks only two elements for validity: that the month is between 0 and 12 and the day is between 0 and 31. MySQL does not automatically verify that the 30th day of the second month (February 30th) is a valid date.
The MySQL date and time datatypes are
DATE A date in YYYY-MM-DD format, between 1000-01-01 and 9999-12-31. For example, December 30th, 1973 would be stored as 1973-12-30.
DATETIME A date and time combination in YYYY-MM-DD HH:MM:SS format, between 1000-01-01 00:00:00 and 9999-12-31 23:59:59. For example, 3:30 in the afternoon on December 30th, 1973 would be stored as 1973-12-30 15:30:00.
TIMESTAMP A timestamp between midnight, January 1, 1970 and sometime in 2037. You can define multiple lengths to the TIMESTAMP field, which directly correlates to what is stored in it. The default length for TIMESTAMP is 14, which stores YYYYMMDDHHMMSS. This looks like the previous DATETIME format, only without the hyphens between numbers; 3:30 in the afternoon on December 30th, 1973 would be stored as 19731230153000. Other definitions of TIMESTAMP are 12 (YYMMDDHHMMSS), 8 (YYYYMMDD), and 6 (YYMMDD).
TIME Stores the time in HH:MM:SS format.
YEAR(M) Stores a year in 2-digit or 4-digit format. If the length is specified as 2 (for example, YEAR(2)), YEAR can be 1970 to 2069 (70 to 69). If the length is specified as 4, YEAR can be 1901 to 2155. The default length is 4.
You will likely use DATETIME or DATE more often than any other date- or time-related data type.
Although numeric and date types are fun, most data you'll store will be in string format. This list describes the common string datatypes in MySQL.
CHAR(M) A fixed-length string between 1 and 255 characters in length (for example, CHAR(5)), right-padded with spaces to the specified length when stored. Defining a length is not required, but the default is 1.
VARCHAR(M) A variable-length string between 1 and 255 characters in length; for example, VARCHAR(25). You must define a length when creating a VARCHAR field.
BLOB or TEXT A field with a maximum length of 65535 characters. BLOBs are "Binary Large Objects" and are used to store large amounts of binary data, such as images or other types of files. Fields defined as TEXT also hold large amounts of data; the difference between the two is that sorts and comparisons on stored data are case sensitive on BLOBs and are not case sensitive in TEXT fields. You do not specify a length with BLOB or TEXT.
TINYBLOB or TINYTEXT A BLOB or TEXT column with a maximum length of 255 characters. You do not specify a length with TINYBLOB or TINYTEXT.
MEDIUMBLOB or MEDIUMTEXT A BLOB or TEXT column with a maximum length of 16777215 characters. You do not specify a length with MEDIUMBLOB or MEDIUMTEXT.
LONGBLOB or LONGTEXT A BLOB or TEXT column with a maximum length of 4294967295 characters. You do not specify a length with LONGBLOB or LONGTEXT.
ENUM An enumeration, which is a fancy term for list. When defining an ENUM, you are creating a list of items from which the value must be selected (or it can be NULL). For example, if you wanted your field to contain "A" or "B" or "C", you would define your ENUM as ENUM ('A', 'B', 'C') and only those values (or NULL) could ever populate that field. ENUMs can have 65535 different values. ENUMs use an index for storing items.
By the Way
The SET type is similar to ENUM in that it is defined as a list. However, the SET type is stored as a full value rather than an index of a value, as with ENUMs.
You will probably use VARCHAR and TEXT fields more often than other field types, and ENUMs are useful as well.