What is the difference between data types var char and text in database design?
The main difference is than TEXT has a fixed max size of 2¹⁶-1 = 65535 characters.
VARCHAR has a variable max size M up to M = 2¹⁶-1.
There are very few differences between VARCHAR and TEXT. Most are not really important.
Summary of *TEXT, CHAR, and VARCHAR:
Never use TINYTEXT.
Almost never use CHAR -- it is fixed length; each character is the max length of the CHARACTER SET (eg, 4 bytes/character for utf8mb4).
With CHAR, use CHARACTER SET ascii unless you know otherwise.
VARCHAR(n) will truncate at n characters; TEXT will truncate at some number of bytes. (But, do you want truncation?)
*TEXT may slow down complex SELECTs due to how temp tables are handled.
VARCHAR column can be given with any size, but it is limited by the maximum size of a single row of data (including all columns), which is 64KB (2¹⁶-1) .TEXT columns do not add to the maximum row size, because the actual text is not stored with the rest of the row.
Related
Why do we have the limitation on datatypes like Char and varchar etc. but not on Integer ?
Why its designed in such a way ?
Eg:
We can define char(8), but we cannot define Int(8) or integer(8). It would have a max of 11 characters saved for it.
Integer have a fixed size of bytes 1-8, depending which kind you have defined.
char has a fixed size of bytes per character so you defined the size of 8 character you get 8 times the size if the character.
As it makes no sense to limit that fixed size if bytes, mysqkl finally kicked it out(in future) and gives now a warning, when you create a integer with a size
What is difference between SET and TEXT column types?
I know that maximum length of SET column type is 64 elements. If I will use TEXT column type can I avoid this limit?
set and text are two completely different things.
A set allows you to specify which values (up to 64) are allowed in a column, and input values that contain any unique combination of these values. text, on the other hand, is just a really long string.
Yes, you can avoid this limit (64 elements), But TEXT type has its limits:
Maximum length (TEXT type): 65,535 (216−1) bytes = 64 KiB
For more information about maximum storage sizes types, see there:
TINYTEXT, TEXT, MEDIUMTEXT, and LONGTEXT maximum storage sizes
I am storing strings and 99.5+% are less than 255 characters, so I store them in a VARCHAR(255).
The thing is, some of them can be 4kb or so. What's the best way to store those?
Option #1: store them in another table with a pointer to the main.
Option #1.0: add an INT column with DEFAULT NULL and the pointer will be stored there
Option #1.1: the pointer will be stored in the VARCHAR(255) column, e.g 'AAAAAAAAAAA[NUMBER]AAAAAAAAAAAA'
Option #2: increase the size of VARCHAR from 255 to 32767
What's the best of the above, Option #1.0, Option #1.1 or Option #2, performance wise?
Increase the size of your field to fit the max size of your string. A VARCHAR will not use the space unless needed.
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.
http://dev.mysql.com/doc/refman/5.0/en/char.html
The MySQL Definition says that VARCHAR(N) will take up to L + 1 bytes if column values require 0 – 255 bytes, L + 2 bytes if values may require more than 255 bytes where L is the length in bytes of the stored string.
So I guess that option #2 is quite okay, because the small strings will still take less space than 32767 bytes.
EDIT:
Also imagine the countless problems options 1.0 and 1.1 would raise when you actually want to query a string without knowing whether it exceeds the length or not.
Option #2 is clearly best. It just adds 1 byte to the size of each value, and doesn't require any complicated joins to merge in the fields from the second table.
ive came across a website who state that the following:
Ex: CountryCode CHAR(3) CHARSET utf8
We are asking for a column with 3 characters exactly. The required storage for this column
will be such that any 3-letter name must fit in. This means (3 characters) times (3 bytes
per character) = 9 bytes of storage. So CHAR and utf8 together may be less than ideal.
VARCHAR behaves better: it only requires as many bytes per character as described above. So
the text "abc" will only require 3 bytes
Do i need for the text 'abc'(with utf8 and char(3)) 3 bytes or 9 bytes?!
Thanks
MySQL's internal structure places CHAR fields directly within the table structure, e.g. A simple table like:
create table foo (
id int
name char(3)
);
would produce an on-disk record that looks like
xxxxccccccccc
^^^^-- 4 bytes of int storage space
^^^^^^^^^ 9 bytes of utf-8 char space
Since MySQL has no way of knowing in advance what kind of text you'll be storing in that char field, it HAS to assume worst-case, and allocates as much space as 3 chars of 'absolutely the longest possible' utf-8 text might take. If it didn't, then an overly long string would overflow the on-disk storage and start scribbling on an adjacent record.
varchar, on the other hand, only has a small 'stub' data section in the table's raw data, and the varchar's contents are stored elsewhere. That means that your varchar(3) will always occupy the same amount of table-space storage, no matter WHAT kind of character set you're using.
Many people say to me that set VARCHAR(100) doesnt make sense. It the same as put 255. I'd like to know why...
That's rubbish. They may be talking about the fact that a varchar uses one byte for the length regardless of whether the maximum length is 100 or 255 ( lengths above that will use two bytes, up to ~64K) but they are treated differently.
If you insert a 150-character string into the former, it will be truncated to 100, that's not so for the latter case.
You should use the length that makes sense. If you have a column that will never exceed 30 characters, don't use varchar(255).
See here for the type details.
http://msdn.microsoft.com/en-us/library/aa258242%28v=sql.80%29.aspx.
VARCHAR(100) does makes sense, it says that the max size of the input is 100 chars(if you will insert a longer string it will cut it to a size 100).
VARCHAR(256) it says that the max size of the input is 256 chars.