By definition:
VARCHAR: The range of Length is 1 to 255 characters. VARCHAR values are sorted and compared in case-insensitive fashion unless the BINARY keyword is given. x+1 bytes
TINYBLOB, TINYTEXT: A BLOB or TEXT column with a maximum length of 255 (2^8 - 1) characters x+1 bytes
So based on this, I creaate the following table:
CREATE TABLE `user` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`name` varchar(255),
`lastname` tinytext,
PRIMARY KEY (`id`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1
Or is it better to create a varchar or tinytext and why?
Is it the same for:
VARCHAR: The range of Length is > 255 characters. VARCHAR values are sorted and compared in case-insensitive fashion unless the BINARY keyword is given. x+2 bytes
BLOB, TEXT A BLOB or TEXT column with a maximum length of 65535 (2^16 - 1) characters x+2 bytes
In this case varchar is better.
Note that varchar can be from 1 to 65535 chars.
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. See Section E.7.4, “Table Column-Count and Row-Size Limits”.
Blobs are saved in a separate section of the file.
They require an extra fileread to include in the data.
For this reason varchar is fetched much faster.
If you have a large blob that you access infrequently, than a blob makes more sense.
Storing the blob data in a separate (part of the) file allows your core data file to be smaller and thus be fetched quicker.
Related
I created this table:
CREATE TABLE Hospital_MedicalRecord(
recNo CHAR(5),
patient CHAR(9),
doctor CHAR(9),
enteredOn DATETIME NOT NULL,
diagnosis LONGTEXT NOT NULL,
treatment TEXT(1000),
PRIMARY KEY (recNo, patient),
CONSTRAINT FK_patient FOREIGN KEY (patient) REFERENCES Hospital_Patient(NINumber),
CONSTRAINT FK_doctor FOREIGN KEY (doctor) REFERENCES Hospital_Doctor(NINumber)
ON DELETE CASCADE
);
How can one make diagnosis contain some long text but never more than 2^24 bytes? I've looked into LONGTEXT but I couldn't find a way to limit it since it can go up to 2^34 I believe?
Use MEDIUMTEXT.
https://dev.mysql.com/doc/refman/8.0/en/string-type-overview.html
MEDIUMTEXT [CHARACTER SET charset_name] [COLLATE collation_name]
A TEXT column with a maximum length of 16,777,215 (224 − 1) characters. The effective maximum length is less if the value contains multibyte characters. Each MEDIUMTEXT value is stored using a 3-byte length prefix that indicates the number of bytes in the value.
The wording is a little strange. The length limit is really on bytes, not characters.
LONGTEXT is limited to the maximum that the filesystem permits for a 32 bit system it is limited to 2^32 that is approximately 4.000.000.000 characters (if you do not use multi-byte characters), I have not calculated exactly.
Then you have MEDIUMTEXT with 2^24 characters .. around 16.000.000 characters. TEXT has a limit at 2^16 character that is much smaller, about 64.000 characters (if you do not have multibyte).
What You need is called MEDIUMTEXT
CREATE TABLE IF NOT EXISTS `test` (
`id` bigint(20) NOT NULL AUTO_INCREMENT,
`country` varchar(5) NOT NULL,
`state` char(5) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 AUTO_INCREMENT=1 ;
I tried following query to insert data
INSERT INTO `test`.`test` (`id` ,`country` ,`state`)
VALUES (NULL , 'south-india', 'Gujarat');
When I execute above query It will shows following warning
Warning: #1265 Data truncated for column 'country' at row 1
Warning: #1265 Data truncated for column 'state' at row 1
I found Reference that VARCHAR is variable-length.CHAR is fixed length.
Then what you mean by
VARCHAR is variable-length.
CHAR is fixed length.
VARCHAR(5) will use at most 5 characters of storage, while CHAR(5) will always use exactly 5.
For a field holding a person's name, for example, you'd want to use a VARCHAR, because while on average someone's name is usually short, you still want to cope with the few people with very long names, without having to have that space wasted for the majority of your database rows.
As you said varchar is variable-length and char is fixed. But the main difference is the byte it uses.
Example.
column: username
type: char(10)
if you have data on column username which is 'test', it will use 10 bytes. and it will have space.
'test______'
Hence the varchar column will only uses the byte you use. for 'test' it will only use 4 bytes. and your data will be
'test'
THanks.
As you mentioned VARCHAR is variable-length. CHAR is fixed length.
when you say
Varchar(5) and if the data you store in it is of length 1, The
remaining 4 byte memory space will be used by others. example: "t"
on the other hand
Char(5) and if the data you store in it is of length 1, The remaining
4 byte memory space cant be used. The 4 byte will end up not used by
any other data. example: "t____" here ____ is the unused space.
I have a table created like so:
CREATE TABLE `my_table` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`info` varchar(50) DEFAULT NULL,
`some_more_info` smallint(5) unsigned NOT NULL
PRIMARY KEY (`id`),
KEY `my_index` (`some_more_info`,`info`(24)),
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8
My question is about the second key called my_index. What does the "(24)" size limit mean? The actual size of the column is 50, but the index is only 24 characters.
Does this mean that MySQL indexes only the first 24 characters of the column info?
In short, yes, the first 24 characters are taken into consideration to build the BTree index. Indexing limits are assigned to text types such as varchar and text, as they don't affect numeric precision.
Yes.
The entire description about the index length can be found here:
http://dev.mysql.com/doc/refman/5.0/en/create-index.html
Prefix lengths are given in characters for nonbinary string types and
in bytes for binary string types. That is, index entries consist of
the first length characters of each column value for CHAR, VARCHAR,
and TEXT columns, and the first length bytes of each column value for
BINARY, VARBINARY, and BLOB columns.
Also you create query has/had some extra ,'s.
I'm just newbie in mysql.
I want to declare my table something like this,
CREATE TABLE `test` (
`NO` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`ID` char(50) CHARACTER SET utf8 NOT NULL ,
`FIRST_NAME` char(50)CHARACTER SET utf8 NOT NULL,
`LAST_NAME` smallint(50)char(50)CHARACTER SET utf8 NOT NULL,
PRIMARY KEY (`NO`),
)
I read in mysql about char and varchar that
The length of a CHAR column is fixed to the length that you declare when you create the table.
My question is that if I use unicode char(50) for chinese, japanese, korean or other unicode characters, would these columns use too much storage in database and can affect the performance?
For english characters, it only can accept up to 50 characters if I declare my table like this?
Is there any better way or is it good to use char(100) for unicode?
Correct me if I'm wrong.
According to the MySQL manual, the number of bytes required by a CHAR(M) column is M x w, where w is "the number of bytes required for the maximum-length character in the character set". This clearly suggests that a CHAR(50) column will store 50 characters, not 50 bytes. If your characters happen to be Chinese, etc., it will still store up to 50 of them. Note that a VARCHAR(50) will also store up to 50 characters, but will (if one ignores the 1-2 byte overhead that comes with a VARCHAR column) take up less storage than a CHAR(50) column when there are fewer than 50 characters stored.
EDIT This applies only if you are using MySQL 5.5 or later. Earlier versions interpreted the length of character and text fields as bytes, not characters.
CREATE SCHEMA IF NOT EXISTS `utftest` DEFAULT CHARACTER SET utf16;
CREATE TABLE IF NOT EXISTS `metadata_labels` (`metadata_id` INT NOT NULL , `label` VARCHAR(256) NOT NULL , PRIMARY KEY (`metadata_id`, `label`));
however I get the following error msg:
Specified key was too long; max key length is 767 bytes
Please advise
UTF 16 uses 32 bits per character (4 bytes) in MySQL. 4 x 256 > 767.
If possible, I would recommend using something other than UTF16 VARCHAR for your key.
In UTF8, it would require 3 x 256 + 4 = 772 bytes. UTF16 would take another 25% more.
You shouldn't use a primary key that's so wide; for an index to be efficient, the storage for each index should be kept to a minimum.
If you need to prevent duplicates, I would recommend adding a calculated field that contains a hash of the contents (e.g. sha1) and create a unique constraint on that instead.
Alternatively, use latin1 as the character encoding for the label field to reduce the number of bytes to 256 + 4 = 300.
If Unicode is a must and hashes are out of the picture you should reduce the column to either UTF8 (250 chars) or UTF16 (190 chars)