转载自：http://www.mysqltutorial.org/mysql-varchar/
The Essential Guide to MySQL VARCHAR Data Type
Summary: this tutorial introduces you to the MySQL VARCHAR data type and discusses some important features of VARCHAR.
Introduction to MySQL VARCHAR data type
MySQL VARCHAR is the variable-length string whose length can be up to 65,535. MySQL stores a VARCHARvalue as a 1-byte or 2-byte length prefix plus actual data.
The length prefix specifies the number of bytes in the value. If a column requires less than 255 bytes, the length prefix is 1 byte. In case the column requires more than 255 bytes, the length prefix is two length bytes.
The maximum length, however, is subject to maximum row size (65,535 bytes) and the character setused. It means that the total length of all columns should be less than 65,535 bytes.
Let’s take a look at an example.
We will create a new table that has two columns s1 and s2 with the length of 32765(+2 for length prefix) and 32766 (+2).Note that 32765+2+32766+2=65535, which is the maximum row size.
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CREATE TABLE IF NOT EXISTS varchar_test (
s1 VARCHAR(32765) NOT NULL,
s2 VARCHAR(32766) NOT NULL
)
CHARACTER SET 'latin1' COLLATE LATIN1_DANISH_CI;
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The statement created the table successfully. However, if we increase the length of the s1 column by 1.
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CREATE TABLE IF NOT EXISTS varchar_test_2 (
s1 VARCHAR(32766) NOT NULL, -- error
s2 VARCHAR(32766) NOT NULL
)
CHARACTER SET 'latin1' COLLATE LATIN1_DANISH_CI;
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MySQL will issue the error message:
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Error Code: 1118. Row size too large. The maximum row size for the used table type, not counting BLOBs, is 65535. This includes storage overhead, check the manual. You have to change some columns to TEXT or BLOBs 0.000 sec
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As you can see, the row size is too large and the statement failed.
If you insert a string whose length is greater than the length of a VARCHAR column, MySQL will issue an error. Consider the following example:
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CREATE TABLE items (
id INT PRIMARY KEY AUTO_INCREMENT,
title VARCHAR(3)
);
INSERT INTO items(title)
VALUES('ABCD');
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In this example, MySQL issued the following error message:
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Error Code: 1406. Data too long for column 'title' at row 1 0.000 sec
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MySQL VARCHAR and spaces
MySQL does not pad space when it stores the VARCHAR values. Also, MySQL retains the trailing spaces when it inserts or selects VARCHAR values. See the following example:
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INSERT INTO items(title)
VALUES('AB ');
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SELECT
id, title, length(title)
FROM
items;
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However, MySQL will truncate the trailing spaces when inserting a VARCHAR value that contains trailing spaces which cause the column length exceeded. In addition, MySQL issues a warning. Let’s see the following example:
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INSERT INTO items(title)
VALUES('ABC ');
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This statement inserts a string whose length is 4 into the title column. MySQL still inserts the string, however, it truncates the trailing space before inserting the value.
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1
row(s) affected, 1 warning(s): 1265 Data truncated for column 'title' at row 1
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You can verify it by using the following query:
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SELECT
title, LENGTH(title)
FROM
items;
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In this tutorial, you have learned how to use MySQL VARCHAR data type to store variable strings in the database.