I read a lot of articles and SO questions about the topic, but all they did was to confuse me more.
I'm trying to understand what is the longest string that a VARCHAR can hold, and how to define it.
In some places it said that a VARCHAR can be created with a max length of 255 (i.e. VARCHAR(255)) - I don't understand if it means 255 bytes or characters.
In other places it said that a VARCHAR can hold up to 8000 bytes - and then depends on the language (if it's 1-byte per character, such as Latin, or more - that determined the length of the longest string).
In simple terms, what does the n in VARCHAR(n) stands for, and what is the range of n?
Is it bytes? Is it the number of a character? between 0-255? between 0-8000?
How does a really long text is saved? does it get split into multiple columns?
VARCHAR is storing strings in 1 byte per symbol (an opposite to nvarchar that can use 2 or more bytes per character). You can read details here.
A common misconception is to think that CHAR(n) and VARCHAR(n), the n defines the number of characters. But in CHAR(n) and VARCHAR(n) the n defines the string length in bytes (0-8,000). n never defines numbers of characters that can be stored. This is similar to the definition of NCHAR(n) and NVARCHAR(n). The misconception happens because when using single-byte encoding, the storage size of CHAR and VARCHAR is n bytes and the number of characters is also n. However, for multi-byte encoding such as UTF-8, higher Unicode ranges (128-1,114,111) result in one character using two or more bytes. For example, in a column defined as CHAR(10), the Database Engine can store 10 characters that use single-byte encoding (Unicode range 0-127), but less than 10 characters when using multi-byte encoding (Unicode range 128-1,114,111). For more information about Unicode storage and character ranges, see Storage differences between UTF-8 and UTF-16.
Related
I was reading the mysql documentation on the byte size for different data types, but was a little confused when it came to char, varchar and decimal.
Can somebody help explain the bytes for these three data types, and also answer how many bytes for the following:
char(7)
varchar(9)
decimal(15,2)
decimal(11,6)
Thanks
CHAR(N) is probably the most confusing because a char is not a fixed byte size across character sets. Furthermore, different row formats handle this problem differently. Tersely, if you're using ROW_FORMAT=COMPACT, ROW_FORMAT=DYNAMIC or ROW_FORMAT=COMPRESSED then CHAR(N) reverse a minimum of N bytes in order to achieve updates in place without fragmentation. If more bytes are required as the result of a different character encoding than it will use more as necessary, trying to use as few as possible, and NO MORE than the maximum character byte length * N is used. If you're using ROW_FORMAT=REDUNDANT, than CHAR(N) always uses the maximum character byte length * N.
VARCHAR(N) and VARBINARY(N) sets a maximum character length per column of N. Below N, MySQL uses the number of bytes required given the string and character encoding used. MySQL then uses one additional byte to record the length of the string if the string is below 256 bytes. If the length of the string is greater than 255 bytes than it uses 2 bytes to record the length of the string. VAR columns are storage efficient but for string columns with frequent UPDATES, one can trade storage for performance by using a fixed length column such as BINARY.
The DECIMAL description is pretty self explanatory:
"Values for DECIMAL (and NUMERIC) columns are represented using a binary format that packs nine decimal (base 10) digits into four bytes. Storage for the integer and fractional parts of each value are determined separately. Each multiple of nine digits requires four bytes, and the “leftover” digits require some fraction of four bytes. The storage required for excess digits is given by the following table"
I come across the below statement while studying about HTML Character Sets and Character Encoding :
Since ASCII used 7 bits for the character, it could only represent 128
different characters.
When we convert any decimal value from the ASCII character set to its binary equivalent it comes down to a 7-bits long binary number.
E.g. For Capital English Letter 'E' the decimal value of 69 exists in ASCII table. If we convert '69' to it's binary equivalent it comes down to the 7-bits long binary number 1000101
Then, why in the ASCII Table it's been mentioned as a 8-bits long binary number 01000101 instead of a 7-bits long binary number 1000101 ?
This is contradictory to the statement
Since ASCII used 7 bits for the character, it could only represent 128
different characters.
The above statement is saying that ASCII used 7 bits for the character.
Please clear my confusion about considering the binary equivalent of a decimal value. Whether should I consider a 7-bits long binary equivalent or a 8-bits long binary equivalent of any decimal value from the ASCII Table? Please explain to me in an easy to understand language.
Again, consider the below statement :
Since ASCII used 7 bits for the character, it could only represent 128
different characters.
According to the above statement how does the number of characters(128) that ASCII supports relates to the fact that ASCII uses 7 bits to represent any character?
Please clear the confusion.
Thank You.
In most processors, memory is byte-addressable and not bit-addressable. That is, a memory address gives the location of an 8-bit value. So, almost all data is manipulated in multiples of 8 bits at a time.
If we were to store a value that has by its nature only 7 bits, we would very often use one byte per value. If the data is a sequence of such values, as text might be, we would still use one byte per value to make counting, sizing, indexing and iterating easier.
When we describe the value of a byte, we often show all of its bits, either in binary or hexadecimal. If a value is some sort of integer (say of 1, 2, 4, or 8 bytes) and its decimal representation would be more understandable, we would write the decimal digits for the whole integer. But in those cases, we might lose the concept of how many bytes it is.
BTW—HTML doesn't have anything to do with ASCII. And, Extended ASCII isn't one encoding. The fundamental rule of character encodings is to read (decode) with the encoding the text was written (encoded) with. So, a communication consists of the transferring of bytes and a shared understanding of the character encoding. (That makes saying "Extended ASCII" so inadequate as to be nearly useless.)
An HTML document represents a sequence of Unicode characters. So, one of the Unicode character encodings (UTF-8) is the most common encoding for an HTML document. Regardless, after it is read, the result is Unicode. An HTML document could be encoded in ASCII but, why do that? If you did know it was ASCII, you could just as easily know that it's UTF-8.
Outside of HTML, ASCII is used billions—if not trillions—of times per second. But, unless you know exactly how it pertains to your work, forget about it, you probably aren't using ASCII.
After reading the documentation, I understood that there is a one-byte or two-byte length prefix to a varying character so as to determine its length. I understand too that, for a varchar, each character might have a different length in bytes depending on the character itself.
So my question is:
How does the DBMS determine each character's length after it's stored?
Meaning: After a string is stored, let's say it's 4 characters long, and let's suppose that the first character is 1 byte long, the second 2 bytes, the 3rd 3 bytes and the 4th is 4..
How does the DB know how long is each character when retrieving the string so as to read it correctly?
I hope the question is clear, sorry for any English mistakes I made. Thanks
The way UTF-8 works as a variable-length encoding is that the 1-byte characters can only use 7 bits of that byte.
If the high bit is 0, then the byte is a 1-byte character (which happens to be encoded in the same way as the 128 ASCII characters).
If the high bit is 1, then it's a multi-byte character.
Picture from https://en.wikipedia.org/wiki/UTF-8
If you're talking about UTF-8, that's not quite how it works. It uses the highest bit in each byte to indicate that the character continues into the next byte, and can store one, two, three or four byte characters fairly efficiently. This is in contrast to UTF-32 where every character is automatically four bytes, something that is obviously very wasteful for some types of text.
When using UTF-8, or any character set where the characters are a variable number of bytes, there's a disconnect between the length of the string in bytes and the length of the string in characters. In a fixed-length system like Latin1, which is rigidly 8-bit, there's no such drift.
Internally the database is most concerned with the length of a field in terms of bytes. The length in terms of characters is only explicitly exposed when calling functions like LENGTH(), as otherwise it's just a bunch of bytes that, if necessary, can be interpreted as a string.
Historically speaking the database stored the length of a field in bytes in a single byte, then the data itself. That's why VARCHAR(255) is so prevalent: It's the longest string you can represent with a single byte length field. Newer databases like Postgres allow >2GB character fields, so they're using four or more bytes to represent the length.
If I create a utf-8 column in MySQL and all the characters are ascii minus 1, how many characters will I be able to store?
Can someone please explain this?
21,844
Will be the maximum for a varchar in utf8.
It's actual chars not bytes.
from MySQL doc
For example, utf8 characters can require up to three bytes per
character, so a VARCHAR column that uses the utf8 character set can be
declared to be a maximum of 21,844 characters. See Section E.7.4,
“Limits on Table Column Count and Row Size”.
MySQL stores VARCHAR values as a 1-byte or 2-byte length prefix plus
data. The length prefix indicates the number of bytes in the value. A
VARCHAR column uses one length byte if values require no more than 255
bytes, two length bytes if values may require more than 255 bytes.
from Wiki
The first 128 characters (US-ASCII) need one byte. The next 1,920
characters need two bytes to encode. This covers the remainder of
almost all Latin alphabets, and also Greek, Cyrillic, Coptic,
Armenian, Hebrew, Arabic, Syriac and Tāna alphabets, as well as
Combining Diacritical Marks. Three bytes are needed for characters in
the rest of the Basic Multilingual Plane (which contains virtually all
characters in common use[11]). Four bytes are needed for characters in
the other planes of Unicode, which include less common CJK characters
and various historic scripts and mathematical symbols.
How many characters can a SQL Server 2008 database field contain when the data type is VARCHAR(MAX)?
From http://msdn.microsoft.com/en-us/library/ms176089.aspx
varchar [ ( n | max ) ]
Variable-length, non-Unicode character
data. n can be a value from 1 through
8,000. max indicates that the maximum
storage size is 2^31-1 bytes. The
storage size is the actual length of
data entered + 2 bytes. The data
entered can be 0 characters in length.
The ISO synonyms for varchar are char
varying or character varying.
1 character = 1 byte. And don't forget 2 bytes for the termination. So, 2^31-3 characters.
For future readers who need this answer quickly:
2^31-1 = 2 147 483 647 characters, or roughly 2.147 billion
See the MSDN reference table for maximum numbers/sizes.
Bytes per varchar(max),
varbinary(max), xml, text, or image
column: 2^31-1
There's a two-byte overhead for the column, so the actual data is 2^31-3 max bytes in length. Assuming you're using a single-byte character encoding, that's 2^31-3 characters total. (If you're using a character encoding that uses more than one byte per character, divide by the total number of bytes per character. If you're using a variable-length character encoding, all bets are off.)
There are a few gotchas worth mentioning - you may need to force the use of varchar(max)
https://dba.stackexchange.com/questions/18483/varcharmax-field-cutting-off-data-after-8000-characters
and print only handles 8000 chars
How to print VARCHAR(MAX) using Print Statement?