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char(??) vs varchar(??)


char(??) vs varchar(??)

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mike 57299
mike 57299
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Hi all.

I have a database with over 200 tables. They all use char(xx) instead of varchar(xx). I heard that varchar is better because it saves space.

I tried to change a table from char(xx) to varchar(xx). It seems that it doubled the amount of space used. What gives?

Does it really matter?

Thanks,
Mike
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Char:

1.Fixed length memory storage
2.CHAR takes up 1 byte per character
3.Use Char when the data entries in a column are expected to be the same size
5.Ex:
Declare test Char(100);
test="Test" -
Then "test" occupies 100 bytes first four bytes with values and rest with blank data.


VarChar:

1.Variable length memory storage(Changeable)
2.VARCHAR takes up 1 byte per character, + 2 bytes to hold length information
3.varchar when the data entries in a column are expected to vary considerably in size.
4.Ex:
Declare test VarChar(100);
test="Test" -
Then "test" occupies only 4+2=6 bytes. first four bytes for value and other two bytes for variable length information.

Conclusion:

1.When Using the fixed length data's in column like phone number, use Char
2.When using the variable length data's in column like address use VarChar

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pls check below code...
declare @var_chr char(50)='abc'
declare @var_varchr varchar(50)='abc'
select DATALENGTH(@var_chr) chr,DATALENGTH(@var_varchr) var

O/P:chr var
50 3
dwain.c
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Or you could construct a query against your table as follows to check total actual string lengths for both scenarios.

Assume the column (Your_Column) in your table (YourTable) is VARCHAR(100):


SELECT VARCHAR=SUM(2 + CAST(DATALENGTH(ISNULL(Your_Column,'')) AS BIGINT))
,CHAR=SUM(CAST(100 AS BIGINT))
FROM YourTable




There's probably also information on this in the sys tables but I don't recall the details.

Edit: Note that this is only an estimate as I'm not 100% sure that NULL values take up the 2 byte length specifier.


My mantra: No loops! No CURSORs! No RBAR! Hoo-uh!

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My advice:
INDEXing a poor-performing query is like putting sugar on cat food. Yeah, it probably tastes better but are you sure you want to eat it?
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Important to remember:


When someone says to use a certain technique because it is better/faster/more economical, make it a point to understand WHY. Blind faith doesn't take you very far in SQL

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There are a couple of factors that can make varchar take more space than char of the same length. First is very short strings in a small column. For example, storing "AA" in char(2) takes 2 bytes, but storing it in varchar(2) takes 4 bytes (because of the length data added to variable-length columns).

The other is ANSI padding. It can do weird things to trailing spaces in varchar() columns. Check out this for details: http://msdn.microsoft.com/en-us/library/ms187403.aspx

For fixed-length data, or for very small columns (up to about 10 characters), I use char() instead of varchar(). Unless the data will much more often be very short than very long. Partially, this is due to fixed-length columns not resulting in page splits when updated to longer values. Page splits often result in extra disk allocation being needed, even if only a small percentage of the pages are actually "full", plus they can result in performance issues in busy databases.

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mike 57299 (10/29/2012)
I tried to change a table from char(xx) to varchar(xx). It seems that it doubled the amount of space used. What gives?


Rebuild the clustered index, the table should drop back down (providing those char columns you changed weren't something like char(2))

Gail Shaw
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CELKO (10/31/2012)

or better:
zip_code CHAR(5) NOT NULL
CHECK (zip_code LIKE '[0-9][0-9][0-9][0-9][0-9]')


As long as whatever that is is intended for US only, now and always. I just love finding web sites and forms that insist I put a 5-digit post code (mine is 4 digits).

Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability

We walk in the dark places no others will enter
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GilaMonster (10/31/2012)
CELKO (10/31/2012)

or better:
zip_code CHAR(5) NOT NULL
CHECK (zip_code LIKE '[0-9][0-9][0-9][0-9][0-9]')


As long as whatever that is is intended for US only, now and always. I just love finding web sites and forms that insist I put a 5-digit post code (mine is 4 digits).


Then you have UK post codes which are 5-8 characters long, depending on whether you include a space or not.

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