When defining a column in SQL Server we can do [Photo] VARBINARY(MAX), this automatically sets the length of the field to the maximum. In MySQL I've tried to do the same but it seems that that this should be done in another way. So, I've been looking for the equivalent of this in MySQL. I've tried googling by I keep coming up with the results defining the max in numeric value like 65535. Tried that, but the MySQL says it is too large for the field.
No in MySQL there's no such a const MAX which defines the maximum column length. Values in VARCHAR columns are variable-length strings. The length can be specified as a value from 0 to 65,535. The effective maximum length of a VARCHAR is subject to the maximum row size (65,535 bytes, which is shared among all columns) and the character set used.
The equivalent to varbinary(max) would LONGBLOB in MySQL:
http://dev.mysql.com/doc/refman/5.5/en/blob.html
Related
I need to create column in mysql 5.1 that can store user's feedback.
It shouldn't be too long, so I think not more 1000 characters of UTF-8.
The question is how to represent this efficiently in mysql 5.1.
For now I have:
`description` varchar NOT NULL,
But how to constrain varchar to hold at most 1000 characters of UTF-8?
From the documentation:
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.
This means that you can store up to 65,535 bytes in a VARCHAR column. However, from the String Type Overview:
MySQL interprets length specifications in character column definitions in character units. (Before MySQL 4.1, column lengths were interpreted in bytes.) This applies to CHAR, VARCHAR, and the TEXT types.
So, declare your table with a UTF8 collation and set the length of the varchar to 1,000 characters and MySQL will do the work for you behind the scenes.
Since the size is apparently defined in bytes, ...
-correction- Field size is defined in 'character units'. It's a bit unclear what they mean by that, but I guess they mean 'code units'.
Removed the rest of the detailed explanation, since it wasn't (entirely true).
Correction. In MySQL you actually define the number of characters in the field. It is still limited to the 65535 byte boundary though. Above that, MySQL just reserves 3 bytes per character for UTF-8, which means that you cannot have UTF-8 fields of more than 21844 characters, and declaring a field als VARCHAR(21900) will just fail for that reason: " Column length too big for column 'field1' (max = 21845); use BLOB or TEXT instead: ". The number in this message is wrong, by the way. The actual maximum size is 21844. 21845 is 1/3 of 65535, but I guess you need to subtract the two bytes for the field size header as well.
The limit of 3 bytes is weird, though. The unicode definition is designed to be able to expand with extra characters. There are already supplementary characters of 4 bytes, that actually cannot be stored in a UTF-8 varchar(1) field, or any varchar field for that matter, since MySQL just doesn't seem able to read those characters: "Incorrect string value: '\xF0\xA0\x9C\x8E' for column 'field1' at row 1". So I guess you would need an actual binary/blob column to be able to store these characters.
I think the documentation about this subject is pretty poor, but I've tried some things and came to this conclusion. You can see the fiddle here: http://sqlfiddle.com/#!2/4d938
To the question:
So for your specific situation, declaring the field as varchar(1000) will do the trick, presuming you don't want people to use the supplementary characters in their feedback.
Some things to consider though:
I think a 'feedback' field of 1000 characters is pretty small. For many folks this will be enough, but if you have to say more, it is annoying if you can't. So I would make the field bigger.
varchar fields are stored in the record and consume a part of the maximum row size of 65536 bytes. This is an important fact. You cannot have two varchar(20000) fields in a row, because together they would be larger than this maximum row size.
A better alternative for large text fields would be therefor be to make them TEXT or MEDIUMTEXT, which can be even larger and are stored in a different way.
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.
http://dev.mysql.com/doc/refman/5.0/en/char.html
In MySQL what is the difference between VARCHAR(1024) and VARCHAR(512)? If my item will never be more than 512 characters, what do I lose by using VARCHAR(1024)?
Don't know where you got that from, but it's not possible to create a table with varchar without specifying the length. It results in a syntax error. So your question is obsolete.
UPDATE:
Nothing. Varchar is as the name implies a datatype of variable length, at least to the maximum length you specified when creating the table. This means, that in a varchar column for each row one additional byte is used to store how long the string in the row actually is.
So the difference between varchar(1024) and varchar(512) is, that your data gets truncated when you try to insert more than 1024 or 512 bytes. Note: bytes, not characters. How much bytes each character uses is dependent on the character set you're using.
There is a actually a difference. And it can have a big performance impact if you manipulate big data. If a temporary table is used, the records on disk will take the full length indicated instead of the variable length. A high value will slow down the request even more in that case. Temporary tables can occur for various reasons (such as memory full, or some combinations of group by /order by).
VARCHAR(1024) 1024 this is lenght.
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.
According to mySQL documentation
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.
A deeper analysis of the performance impact of larger VARCHARs can be found here.
My server has my SQL version of 5.0.91-community, now i have to store a long string of approx about 500 character more or less, i thought of going with text data type but then someone told me it slows the performance, i wanted to know more about varchar and it's limit.
i used to think that varchar is only limited to 255 characters, but then i read it somewhere it is capable of storing more then that in the newer version i.e >= 5.0.3 , as i am using 5.0.91 what do you think i should use? if i use it like varchar(1000) is it still valid?
thank you.
The documentation is here,
varchar has a max size of 65,535 in MySQL 5.0.3 and later , before 5.0.3 the limit was 255
Note that the effective size is less,
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.
You have to specify the max size, e.g. varchar(1000). Just stating varchar isn't enough.
From The CHAR and VARCHAR Types
Values in VARCHAR columns are
variable-length strings. The length
can be specified as a value from 0 to
65,535. The effective maximum length
of a VARCHAR is subject to the maximum
row size (65,535 bytes, which is
shared among all columns) and the
character set used.
According to the MySQL doc:
TEXT differs from VARCHAR in the following ways:
There is no trailing-space removal for TEXT columns when values are stored or retrieved. Before MySQL 5.0.3, this differs from VARCHAR, for which trailing spaces are removed when values are stored.
For indexes on TEXT columns, you must specify an index prefix length. For CHAR and VARCHAR, a prefix length is optional.
TEXT columns cannot have DEFAULT values.
Apart from these differences, using VARCHAR like using TEXT, so the question of size is not what should make you choose between those two, unless you really need to store no more characters than 1000.
In MySQL, VARCHAR accepts maximum of 65535 chars.
You can assure yourself very easy. Mysql documentation is openly accessed and it says
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
as for the performance issues, it doesn't matter. Not data type but data relations affect performance.
If I have a column in table with field of type VARCHAR(15) and if I try to insert data of length 16, MySQL gives an error stating
Data too long for column 'testname' at row 1
Does anyone know why VARCHAR fields in MySQL take fixed length? Also how many bytes does a VARCHAR field take per record based on the size given?
From the MySQL 5.0 Manual:
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.
I only use VARCHAR when I'm certain that the data the column needs to hold will never exceed a certain length, and even then I'm cautious. If I'm storing a text string I tend to use one of the TEXT types.
Check out the MySQL Storage Requirements for more information on how the bytes are used.
If you set a column to be varchar(15) the maximum bytes allowed is 15. Thus you can't pass it more than 15 characters without modifying the column to support more than 15. If you store a 4 character string it should only use around 4 bytes out of a possible 15, whereas if you used char(15) it would have filled in the other 11 with empty bytes.
http://dev.mysql.com/doc/refman/5.0/en/char.html
( My byte calculation was probably off since it's always -1/+1 or something like that ).
Small extra local note. The number of bytes used will depend on the encoding scheme in use. 1 byte per character in latin1 encoding, but up to 3 in UTF8. See link in mlambie's answer for details.
If you look here it should tell you everything about varchar you want to know:
http://dev.mysql.com/doc/refman/5.0/en/char.html
Basically, depending on the length you chose it will use 1 or two bytes to track the length of the current string in that column, so it will store the number of bytes for the data you put in, plus one or two bytes.
So, if you put in 'abc' then it will be 4 or 5 bytes used for that column in that row.
If you used char(15) then even 'abc' would take up 15 bytes, as the data is the right-padded to use up the full 15 bytes.
I am changing my db so that certain fields can be longer. Right now they're "varchar(255)" can I just increase that to "varchar(500)" or are there special rules for dealing with this and I should make them "text"?
From the MySQL manual:
"The effective maximum length of a VARCHAR is subject to the maximum row size (65,535 bytes, which is shared among all columns) and the character set used."
So, yes, you can just extend your columns to be a maximum of 500 characters in length (though, you'll want to be sure your application logic doesn't expect the maximum length to be 255 characters).
Edit: see also cballou's note below about character sets.
Modifying to a longer varchar would be fine:
ALTER TABLE my_table MODIFY column_name VARCHAR(500);