I can't understand why should we set field length while creating SQL table fields.
INT(11),
VARCHAR(255),
Data lengths are another form of data validation - they allow you to easily constrain the data in the table and not allow values that are larger than your program's logic would allow.
These are two different cases.
INT(11)
The "length" of an integer is almost meaningless, and MySQL 8.0 deprecates this syntax. An INT is always a 32-bit integer, regardless of the length argument. The only practical use of the length argument is if you use:
INT(11) ZEROFILL
This pads the number with zeroes when you fetch it, not as it is stored. The number is still stored exactly the same as INT(2) or INT(50), as a 32-bit integer. See my answer to Types in MySQL: BigInt(20) vs Int(20)
VARCHAR(255)
It's necessary to define a length because that's how data types must work in relational theory. The definition of a data type is "a named, finite set of values." It can't be a finite set if the strings have infinite length.
There is also the practical reason: to store a string, MySQL precedes it with one or two bytes encoding the length, so it knows how many characters to read. One byte of length information is used if the length is up to 255. Two bytes of length information is used if the length is up to 65535. That's the maximum length supported for VARCHAR in MySQL.
Related
We know that varchar is a variable length data type then what is the difference between varchar(3) and varchar(300) in mysql ???
In SQL varchar is a string which varies in length. Traditionally, you specify an upper limit for this string. Here are some things to know about varchar:
Strings which are shorter than the specified limit do not take extra space: they only take up the required space.
If the string is longer than the limit, the whole record is rejected, both when you try to INSERT a record, and when you try to UPDATE a record.
Traditionally the upper upper limit was 255 characters. Modern databases no longer have this limit.
Some databases, such as PostgreSQL recommend that you no longer specify the length this way. Instead they recommend an unlimited string (varchar without the length) and limit it using a CHECK CONSTRAINT instead.
Most modern databases handle strings much more efficiently than in the past, so there is less need to be restrictive about the size of the string.
The short answer to your question is that both varchar(3) and varchar(300) are limited-length strings, and both will allow shorter strings without penalty. However clearly one is much shorter than the other.
In addition to #Manngo's answer, varchar(300) has an extra byte of overhead than varchar(3) because its max is over 255. From the MySQL docs...
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.
Basically, the length of the string must be stored. 1 byte can only hold 0 to 255, so 2 bytes are required to store a length that can go up to 300.
Suppose I want to insert a single character in my column in that scenario what is difference between both ?? Why we use varchar(1) why not varchar(100)??
If the column is only supposed to ever store a single character, use char(1), a single byte, to ensure the integrity of the data (varchar(1) is two bytes). That ensures anyone using the column will only ever get what they expect and don't have to do their own data validation.
Since the size of a varchar is only a max, specifying a smaller one won't make the table use any less disk (except as noted above about >255). Avoid adding arbitrary limits and business rules to your columns. For example, if you're going to store a URL or email address, there's little reason not to allow varchar(255). Limits based on business rules and UX concerns should be handed at a higher layer and not limited by the database schema.
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.
I'm using phpMyAdmin to create my table structures.
I can read from the documentation pages on MySQL about size limits for Integer Types:
MySQL Integer Types Reference
So here is where I'm getting a little confused with creating a column.
I want to create a column in the table: tbl_note_categories called notescounter
I don't foresee myself creating thousands of noteids in the tbl_notes with any specific categoryid. But I do believe I'd create hundreds of notes to each categoryid.
I'm at that point of choosing between: tinyint, smallint, mediumint.
According the documentation link above, I'm guessing smallint is my best choice.
So here's my confusion. PhpMyAdmin asks for a Length/Values parameter to be specified.
I'm going to make sure this new column (notescounter) is unsigned, giving me up to 65536.
Does that mean I need the Length/Values to be (5)?
I'm guessing Length is character length, but I'm not sure. (comparing to varchar)
No, this is a common misconception about MySQL. In fact, the "length" has no effect on the size of an integer or the range of values it can store.
TINYINT is always 8 bits and can store 28 distinct values.
SMALLINT is always 16 bits and can store 216 distinct values.
INT is always 32 bits and can store 232 distinct values.
BIGINT is always 64 bits and can store 264 distinct values.
There's also a MEDIUMINT, but the engineers who work on MySQL tell me MEDIUMINT always gets promoted to a 32-bit INT internally, so there's actually no benefit to using MEDIUMINT.
The length is only for display, and this only matters if you use the ZEROFILL option.
See an example in my answer to What is the difference (when being applied to my code) between INT(10) and INT(12)?
Yes, you want to specify a length of 5.
In MySQL, the "length" attribute on the integer types is optional. It's a MySQL extension which is non-standard).
When it is omitted from the column declaration, MySQL provides a default value. For a SMALLINT UNSIGNED, the default value is 5.
This value does NOT have any impact on the range of values that can be stored for an integer type. It specifies a "display length", which is returned in resultset metadata, which a client can choose to use or ignore.
http://dev.mysql.com/doc/refman/5.5/en/numeric-type-attributes.html
I have a general question about this. There are many times we want to change data-types of fields or collations when lots of data is inserted before. Consider these situations :
converting varchar collation from utf8_general_ci to latin1_swedish_ci: as I know the first has multibyte chars and the second singly byte ones. Does this conversion manipulate stored records correctly? And does this conversion lead to reduction of volume of existing data (maybe 50%)?
Conversion of int(10) to smallint(5): Does the volume of data reduce to 50% correctly?
Or for example: int(10) to unsigned int(10) - text to varchar(1000) - varchar(20) to char(10) , ...
As it is obvious, these actions might be done to increase efficiency, reduce volume of data and ...
Consider I have a table with 1,000,000 records. I want to know if doing such actions have bad effects on stored data, or if it makes low performance for future inserts and selects involving this table.
UPDATE :
When I talk about changing utf8 encoding charset to Latin, of course the values of my field are English (it's obvious if there are Japanese, they will be lost). With this assumption, I'm asking about the resulting table size and performance.
Converting varchar collation from utf8_general_ci to latin1_swedish_ci: As I know the first has multibyte chars and the second singly byte ones. Does this conversion manipulate stored records correctly? And does this conversion lead to reduction of volume of existing data (maybe 50%)?
Collation is merely the ordering that is used for string comparisons—it has (almost) nothing to do with the character encoding that is used for data storage. I say almost because collations can only be used with certain character sets, so changing collation may force a change in the character encoding.
To the extent that the character encoding is modified, MySQL will correctly re-encode values to the new character set whether going from single to multi-byte or vice-versa. Beware that any values that become too large for the column will be truncated.
Provided that the new character type is of variable-length and that the values are encoded with fewer bytes in the new encoding than before, there will of course be a reduction in the table's size.
Conversion of int(10) to smallint(5): Does the volume of data reduce to 50% correctly?
INT and SMALLINT respectively occupy 4 and 2 bytes regardless of display width: so yes, the size of the table will reduce accordingly.
Or for example: int(10) to unsigned int(10) - text to varchar(1000) - varchar(20) to char(10), ...
INT occupies 4 bytes irrespective of whether it is signed, so there will be no change;
TEXT and VARCHAR(1000) both occupy L+2 bytes (where L is the value's length in bytes), so there will be no change;
VARCHAR(20) occupies L+1 bytes (where L is the value's length in bytes) whereas CHAR(10) occupies 10×w bytes (where w is the number of bytes required for the maximum-length character in the character set), so there may well be a change but it is dependent on the actual values stored and the character encoding used.
Note that, depending on storage engine, reductions in table size may not immediately be released to the filesystem.
A1. collation does not change your data. it changes sort order in your queries, and possibly changes your indices (?).
A2. length of the data in the column will be reduced, however you always have some overhead per table row, and you cannot change that. moreover, if your data is not unique, you will not see much reduction in index size, because your index looks like this: 33->{row1,row2,row3...},67->{row9,row0,row7} and every row pointer is much larger than an int.
in other words, if you had a table with a hundred int rows, without many indices, and changed all these columns to tinyint, you would see a notable improvement. if it is only one column, don't bother.
http://dev.mysql.com/doc/refman/5.0/en/storage-requirements.html
http://dev.mysql.com/doc/refman/5.0/en/innodb-physical-record.html
A3. please read up on text vs varchar. the earlier stores data separately from table row, the latter in the row. each has own implications.
p.s. row and index overhead depends a lot on what db engine you use. normally you should use innodb. however for read-only tasks, e.g. data mining, myisam is more efficient.
Converting varchar collation from utf8_general_ci to latin1_swedish_ci: It can reduce table(file) size, but you can lose not latin symbols, only english words will be stored correctly.
Conversion of int(10) to smallint(5) - it will reduce the volume of data. Conversion of int(10) to unsigned int(10) - it won't reduce. In these cases you should care about the values, you can get an error - out of range value.
Conversion varchar(20) to char(10): CHARs are used for strings that always have the same length (for example - 10), if the strings are different in length, then use VARCHAR data type.
Im new to Database programming and I have a very basic question:
In my PHPMyAdmin GUI that Im using to create tables in my database, what does it mean when the column "type" (ie. datatype) has the data type and something in brackets after that.
For example:
int(20), bigint(30) .....
I understand the type int and bigint imply the number of bytes that are used and consequently the range of values that can be stored. But what does the value in the brackets mean?
What does the (20) and the (30) stand for.... what impact does this have on....
Sorry if the Q is basic, I am trying to understand databases....
Thanks a lot
Basically this is a Display Width.
I've found very good explanation of this concept here is so decided to not describe it myself and let you read it yourself from the original source.
In the same way that a max-length can be specified for string data types (e.g. VARCHAR(5) = Maximum 5 Characters), Numeric data type cells can have a "Display Length" specified ( E.g.: INT(5) ).
There is a common misconception that specifying a Display Length on an INT column will limit that column's range. As example, it is quite often thought that defining a column as INT(1) will reduce the column's unsigned range to 0 - 9, and that INT(2) would reduce the column's unsigned range to 0 - 99. This is not the case. An INT data column will ALWAYS have a viable unsigned range of 0 - 4294967295, or a signed range of -2147483648 to 2147483647, irrespective of the specified Display Width, whether it be 1 ( INT(1) ) or 20 ( INT(20) ).
Display width doesn't change storage requirements for a data type.
Display width doesn't alter the actual data in any way (ie: it stores the entire value for the data)
A column returns it's full value when called in a query, regardless of the display width (the book directly contradicts this claim it makes as seen above)
The value in the bracket is the size or length of the field. [Edit strike]If set to 2 a uint field can only host values from 0 to 99.[/strike] You can set this value on your own and thus save a bit of memory if you expect your values not to exceed this limitation. Useful in connection with varchar.
Here another thread about varchar sizes: What are the optimum varchar sizes for MySQL?
Link to the mysql doc which explains it http://dev.mysql.com/doc/refman/5.7/en/numeric-type-attributes.html