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SQL - what is difference between char (15) and varchar(15)

Posted on 2012-09-16
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Last Modified: 2012-09-16
Can one explain the difference
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Question by:tommym121
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by:kiwistag
kiwistag earned 1000 total points
ID: 38403781
Effectively varchar(x) can be a value UP to x whereas char(x) pads the table entry to be a total of x - therefore varchar is more efficient whereas char is an exact amount.

From http://dev.mysql.com/doc/refman/5.0/en/char.html:
 The CHAR and VARCHAR types are similar, but differ in the way they are stored and retrieved. As of MySQL 5.0.3, they also differ in maximum length and in whether trailing spaces are retained.

The CHAR and VARCHAR types are declared with a length that indicates the maximum number of characters you want to store. For example, CHAR(30) can hold up to 30 characters.

The length of a CHAR column is fixed to the length that you declare when you create the table. The length can be any value from 0 to 255. When CHAR values are stored, they are right-padded with spaces to the specified length. When CHAR values are retrieved, trailing spaces are removed.

Values in VARCHAR columns are variable-length strings. The length can be specified as a value from 0 to 255 before MySQL 5.0.3, and 0 to 65,535 in 5.0.3 and later versions. The effective maximum length of a VARCHAR in MySQL 5.0.3 and later is subject to the maximum row size (65,535 bytes, which is shared among all columns) and the character set used. See Section E.7.4, “Table Column-Count and Row-Size Limits”.

In contrast to CHAR, VARCHAR values are stored as a 1-byte or 2-byte length prefix plus data. The length prefix indicates the number of bytes in the value. A column uses one length byte if values require no more than 255 bytes, two length bytes if values may require more than 255 bytes.
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Jerry Miller earned 1000 total points
ID: 38403978
In MS SQL, I only use CHAR data types when I know for a fact that the data is a set length will not change in the future. If there is a sliver of doubt as to whether it will change, I use VARCHAR.

When querying the database, using CHAR data types make it a little harder to compare strings since you have to TRIM all of you data as it comes from the table to make sure that you are removing all trailing spaces. If you forget in one query, you will get unexpected results.
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by:tommym121
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