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.
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 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.
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.
I'm new to web programming and doing different tutorials that I can find on the net.
I did my research and found out that in int(11), 11 is the maximum display width for integers and it's the default value if unless the integer is UNSIGNED (in this case it's 10).
When I see something like this:
id INT(11) not null AUTO_INCREMENT
I have no questions. But why do I see different things in different tutorials? For examlpe, in some, it says,
id INT(10) not null AUTO_INCREMENT
And even
id INT(4) not null AUTO_INCREMENT
What are the authors of those tuts trying to achieve? None of them seem to bother to give an explanation what 10 or 4 means.
Alright, they're obviously reducing the display width, but why? What's wrong with the default width of 11? Why would they want to change it? Or are there any other reasons I don't know of?
Thanks.
The x in INT(x) has nothing to do with space requirements or any other performance issues, it's really just the display width. Generally setting the display widths to a reasonable value is mostly useful with the UNSIGNED ZEROFILL option.
//INT(4) UNSIGNED ZEROFILL
0001
0002
...
0099
...
0999
...
9999
...
10000
//INT(2) UNSIGNED ZEROFILL
01
02
...
09
...
99
...
100
Without the UNSIGNED ZEROFILL option the value will be left-padded with spaces to the appropriate display width.
//INT(4)
1
2
...
99
...
999
...
9999
...
10000
//INT(2)
1
2
...
9
...
99
...
100
The number is just a representational option called "display width". It may be used by some applications to pad the field when displaying numeric datatypes.
The int size is neither bits nor bytes. It's just the display width, that is used when the field has ZEROFILL specified.
This blog explains the meaning of int(size) in MySQL.
From the docs:
"This optional display width may be used by applications to display integer values having a width less than the width specified for the column by left-padding them with spaces. (That is, this width is present in the metadata returned with result sets. Whether it is used or not is up to the application.)" That is, int(4) is politely asking that 123 be displayed as " 123". Don't know how many things actually pay attention or why they'd bother in a tutorial.
"The display width does not constrain the range of values that can be stored in the column. Nor does it prevent values wider than the column display width from being displayed correctly." So if you shove 123456 into an int(4) it will still be 123456.
Nothing to do with disk space or performance, just a hint to the application retrieving the data that it would like to be displayed with at least N digits.
The related ZEROFILL option can be used in conjunction to actually pad out the returned data. So 123 in an int(4) ZEROFILL is returned as 0123.
It's not performance increase neither difference in maximum allowed size.
It's only used when ZEROFILL is applied on that column. (See: http://alexander.kirk.at/2007/08/24/what-does-size-in-intsize-of-mysql-mean/)
Yes it specifies the display width, and this comes into play only when ZEROFILL is specified. So at that time it can pad the field value with the required number of zeros.
I agree that the MySQL manual is a little vague on the length/display width of integers. You might think that this limits the value you can store in the column to the number of digits but the valid range of the colomn doesnt change, wether you set it to 1 or 11.
What it really does is determine the display width of the column's value. The only case this is usefull is if you use ZEROFILL to pad your values with zero's.
So basically there is no real difference between INT(1) and INT(11) in terms of what you can store in the column, the only case when it becomes relevant is when you want to ZEROFILL your values.
More info:
http://alexander.kirk.at/2007/08/24/what-does-size-in-intsize-of-mysql-mean/
http://dev.mysql.com/doc/refman/5.0/en/numeric-types.html
Copying from this blog article:
MySQL has a little know feature for numerical types known as zerofill. This feature effects the display size of numerical types. Unlike the string types the number inside the parentheses is not the storage size in characters for the type. For numerical types the type name itself determines storage size.
The integer type it’s the padding size for zerofill.