数据库字符集的原理与常见问题MySQL 5.1 Reference Manual

13.1.2. ALTER TABLE Syntax

ALTER [IGNORE] TABLE tbl_name
    alter_specification [, alter_specification] ...

alter_specification:
    ADD [COLUMN] column_definition [FIRST | AFTER col_name ]
  | ADD [COLUMN] (column_definition,...)
  | ADD {INDEX|KEY} [index_name] [index_type] (index_col_name,...)
  | ADD [CONSTRAINT [symbol]]
        PRIMARY KEY [index_type] (index_col_name,...)
  | ADD [CONSTRAINT [symbol]]
        UNIQUE [INDEX|KEY] [index_name] [index_type] (index_col_name,...)
  | ADD FULLTEXT [INDEX|KEY] [index_name] (index_col_name,...)
      [WITH PARSER parser_name]
  | ADD SPATIAL [INDEX|KEY] [index_name] (index_col_name,...)
  | ADD [CONSTRAINT [symbol]]
        FOREIGN KEY [index_name] (index_col_name,...)
        [reference_definition]
  | ALTER [COLUMN] col_name {SET DEFAULT literal | DROP DEFAULT}
  | CHANGE [COLUMN] old_col_name column_definition
        [FIRST|AFTER col_name]
  | MODIFY [COLUMN] column_definition [FIRST | AFTER col_name]
  | DROP [COLUMN] col_name
  | DROP PRIMARY KEY
  | DROP {INDEX|KEY} index_name
  | DROP FOREIGN KEY fk_symbol
  | DISABLE KEYS
  | ENABLE KEYS
  | RENAME [TO] new_tbl_name
  | ORDER BY col_name
  | CONVERT TO CHARACTER SET charset_name [COLLATE collation_name]
  | [DEFAULT] CHARACTER SET charset_name [COLLATE collation_name]
  | DISCARD TABLESPACE
  | IMPORT TABLESPACE
  | table_option ...
  | partition_options
  | ADD PARTITION (partition_definition)
  | DROP PARTITION partition_names
  | COALESCE PARTITION number
  | REORGANIZE PARTITION partition_names INTO (partition_definitions)
  | ANALYZE PARTITION partition_names
  | CHECK PARTITION partition_names
  | OPTIMIZE PARTITION partition_names
  | REBUILD PARTITION partition_names
  | REPAIR PARTITION partition_names
  | REMOVE PARTITIONING

ALTER TABLE enables you to change the structure of an existing table. For example, you can add or delete columns, create or destroy indexes, change the type of existing columns, or rename columns or the table itself. You can also change the comment for the table and type of the table.

The syntax for many of the allowable alterations is similar to clauses of the CREATE TABLE statement. This includes table_option modifications, for options such as ENGINE, AUTO_INCREMENT, and AVG_ROW_LENGTH. (However, ALTER TABLE ignores the DATA DIRECTORY and INDEX DIRECTORY table options.) Section 13.1.7, “CREATE TABLE Syntax”, lists all table options. As of MySQL 5.1.11, to prevent inadvertant loss of data, ALTER TABLE cannot be used to change the storage engine of a table to MERGE or BLACKHOLE.

Some operations may result in warnings if attempted on a table for which the storage engine does not support the operation. These warnings can be displayed with SHOW WARNINGS. See Section 13.5.4.30, “SHOW WARNINGS Syntax”.

In most cases, ALTER TABLE works by making a temporary copy of the original table. The alteration is performed on the copy, and then the original table is deleted and the new one is renamed. While ALTER TABLE is executing, the original table is readable by other clients. Updates and writes to the table are stalled until the new table is ready, and then are automatically redirected to the new table without any failed updates.

In some cases, no temporary table is necessary:

If you use ALTER TABLE tbl_name RENAME TO new_tbl_name without any other options, MySQL simply renames any files that correspond to the table tbl_name. (You can also use the RENAME TABLE statement to rename tables. See Section 13.1.16, “RENAME TABLE Syntax”.)
ALTER TABLE ... ADD PARTITION creates no temporary table except for MySQL Cluster. ADD or DROP operations for RANGE or LIST partitions are immediate operations or nearly so. ADD or COALESCE operations for HASH or KEY partitions copy data between changed partitions; unless LINEAR HASH/KEY was used, this is much the same as creating a new table (although the operation is done partition by partition). REORGANIZE operations copy only changed partitions and do not touch unchanged ones.

If other cases, MySQL creates a temporary table, even if the data wouldn't strictly need to be copied (such as when you change the name of a column). For MyISAM tables, you can speed up the index re-creation operation (which is the slowest part of the alteration process) by setting the myisam_sort_buffer_size system variable to a high value.

To use ALTER TABLE, you need ALTER, INSERT, and CREATE privileges for the table.
IGNORE is a MySQL extension to standard SQL. It controls how ALTER TABLE works if there are duplicates on unique keys in the new table or if warnings occur when strict mode is enabled. If IGNORE is not specified, the copy is aborted and rolled back if duplicate-key errors occur. If IGNORE is specified, only the first row is used of rows with duplicates on a unique key, The other conflicting rows are deleted. Incorrect values are truncated to the closest matching acceptable value.
You can issue multiple ADD, ALTER, DROP, and CHANGE clauses in a single ALTER TABLE statement, separated by commas. This is a MySQL extension to standard SQL, which allows only one of each clause per ALTER TABLE statement. For example, to drop multiple columns in a single statement, do this:
```
ALTER TABLE t2 DROP COLUMN c, DROP COLUMN d;
```
CHANGE col_name, DROP col_name, and DROP INDEX are MySQL extensions to standard SQL.
MODIFY is an Oracle extension to ALTER TABLE.
The word COLUMN is optional and can be omitted.
column_definition clauses use the same syntax for ADD and CHANGE as for CREATE TABLE. Note that this syntax includes the column name, not just its data type. See Section 13.1.7, “CREATE TABLE Syntax”.
You can rename a column using a CHANGE old_col_name column_definition clause. To do so, specify the old and new column names and the type that the column currently has. For example, to rename an INTEGER column from a to b, you can do this:
```
ALTER TABLE t1 CHANGE a b INTEGER;
```
If you want to change a column's type but not the name, CHANGE syntax still requires an old and new column name, even if they are the same. For example:
```
ALTER TABLE t1 CHANGE b b BIGINT NOT NULL;
```
You can also use MODIFY to change a column's type without renaming it:
```
ALTER TABLE t1 MODIFY b BIGINT NOT NULL;
```
If you use CHANGE or MODIFY to shorten a column for which an index exists on the column, and the resulting column length is less than the index length, MySQL shortens the index automatically.
When you change a data type using CHANGE or MODIFY, MySQL tries to convert existing column values to the new type as well as possible.
To add a column at a specific position within a table row, use FIRST or AFTER col_name. The default is to add the column last. You can also use FIRST and AFTER in CHANGE or MODIFY operations.
ALTER ... SET DEFAULT or ALTER ... DROP DEFAULT specify a new default value for a column or remove the old default value, respectively. If the old default is removed and the column can be NULL, the new default is NULL. If the column cannot be NULL, MySQL assigns a default value, as described in Section 11.1.4, “Data Type Default Values”.
DROP INDEX removes an index. This is a MySQL extension to standard SQL. See Section 13.1.11, “DROP INDEX Syntax”.
If columns are dropped from a table, the columns are also removed from any index of which they are a part. If all columns that make up an index are dropped, the index is dropped as well.
If a table contains only one column, the column cannot be dropped. If what you intend is to remove the table, use DROP TABLE instead.
DROP PRIMARY KEY drops the primary index. Note: In older versions of MySQL, if no primary index existed, DROP PRIMARY KEY would drop the first UNIQUE index in the table. This is not the case in MySQL 5.1, where trying to use DROP PRIMARY KEY on a table with no primary key give rises to an error.

If you add a UNIQUE INDEX or PRIMARY KEY to a table, it is stored before any non-unique index so that MySQL can detect duplicate keys as early as possible.
ORDER BY enables you to create the new table with the rows in a specific order. Note that the table does not remain in this order after inserts and deletes. This option is useful primarily when you know that you are mostly to query the rows in a certain order most of the time. By using this option after major changes to the table, you might be able to get higher performance. In some cases, it might make sorting easier for MySQL if the table is in order by the column that you want to order it by later.
If you use ALTER TABLE on a MyISAM table, all non-unique indexes are created in a separate batch (as for REPAIR TABLE). This should make ALTER TABLE much faster when you have many indexes.

This feature can be activated explicitly. ALTER TABLE ... DISABLE KEYS tells MySQL to stop updating non-unique indexes for a MyISAM table. ALTER TABLE ... ENABLE KEYS then should be used to re-create missing indexes. MySQL does this with a special algorithm that is much faster than inserting keys one by one, so disabling keys before performing bulk insert operations should give a considerable speedup. Using ALTER TABLE ... DISABLE KEYS requires the INDEX privilege in addition to the privileges mentioned earlier.

ENABLE KEYS and DISABLE KEYS were not supported for partitioned tables prior to MySQL 5.1.11. (Bug#19502)
The FOREIGN KEY and REFERENCES clauses are supported by the InnoDB storage engine, which implements ADD [CONSTRAINT [symbol]] FOREIGN KEY (...) REFERENCES ... (...). See Section 15.2.6.4, “FOREIGN KEY Constraints”. For other storage engines, the clauses are parsed but ignored. The CHECK clause is parsed but ignored by all storage engines. See Section 13.1.7, “CREATE TABLE Syntax”. The reason for accepting but ignoring syntax clauses is for compatibility, to make it easier to port code from other SQL servers, and to run applications that create tables with references. See Section 1.9.5, “MySQL Differences from Standard SQL”.

You cannot add a foreign key and drop a foreign key in separate clauses of a single ALTER TABLE statement. You must use separate statements.
InnoDB supports the use of ALTER TABLE to drop foreign keys:
```
ALTER TABLE tbl_name DROP FOREIGN KEY fk_symbol;
```
You cannot add a foreign key and drop a foreign key in separate clauses of a single ALTER TABLE statement. You must use separate statements.

For more information, see Section 15.2.6.4, “FOREIGN KEY Constraints”.
If you want to change the table default character set and all character columns (CHAR, VARCHAR, TEXT) to a new character set, use a statement like this:
```
ALTER TABLE tbl_name CONVERT TO CHARACTER SET charset_name;
```
Warning: The preceding operation converts column values between the character sets. This is not what you want if you have a column in one character set (like latin1) but the stored values actually use some other, incompatible character set (like utf8). In this case, you have to do the following for each such column:
```
ALTER TABLE t1 CHANGE c1 c1 BLOB;
ALTER TABLE t1 CHANGE c1 c1 TEXT CHARACTER SET utf8;
```
The reason this works is that there is no conversion when you convert to or from BLOB columns.

If you specify CONVERT TO CHARACTER SET binary, the CHAR, VARCHAR, and TEXT columns are converted to their corresponding binary string types (BINARY, VARBINARY, BLOB). This means that the columns no longer will have a character set and a subsequent CONVERT TO operation will not apply to them.

To change only the default character set for a table, use this statement:
```
ALTER TABLE tbl_name DEFAULT CHARACTER SET charset_name;
```
The word DEFAULT is optional. The default character set is the character set that is used if you do not specify the character set for a new column which you add to a table (for example, with ALTER TABLE ... ADD column).
For an InnoDB table that is created with its own tablespace in an .ibd file, that file can be discarded and imported. To discard the .ibd file, use this statement:
```
ALTER TABLE tbl_name DISCARD TABLESPACE;
```
This deletes the current .ibd file, so be sure that you have a backup first. Attempting to access the table while the tablespace file is discarded results in an error.

To import the backup .ibd file back into the table, copy it into the database directory, and then issue this statement:
```
ALTER TABLE tbl_name IMPORT TABLESPACE;
```
See Section 15.2.3.1, “Using Per-Table Tablespaces”.
A number of partitioning-related extensions to ALTER TABLE were added in MySQL 5.1.5. These can be used with partitioned tables for repartitioning, for adding, dropping, merging, and splitting partitions, and for performing partitioning maintenance.

The partition_definition clause for ALTER TABLE ADD PARTITION supports the same options as the clause of the same name does for the CREATE TABLE statement clause of the same name. (See Section 13.1.7, “CREATE TABLE Syntax”, for the syntax and description.) Suppose that you have the partitioned table created as shown here:
```
CREATE TABLE t1 (
    id INT,
    year_col INT
)
PARTITION BY RANGE (year_col) (
    PARTITION p0 VALUES LESS THAN (1991),
    PARTITION p1 VALUES LESS THAN (1995),
    PARTITION p2 VALUES LESS THAN (1999)
);
```
You can add a new partition p3 to this table for storing values less then 2002 as follows:
```
ALTER TABLE t1 ADD PARTITION (PARTITION p3 VALUES LESS THAN (2002));
```
Note: You cannot use ALTER TABLE to add partitions to a table that is not already partitioned.

DROP PARTITION can be used to drop one or more RANGE or LIST partitions. This statement cannot be used with HASH or KEY partitions; instead, use COALESCE PARTITION (see below). Any data that was stored in the dropped partitions named in the partition_names list is discarded. For example, given the table t1 defined previously, you can drop the partitions named p0 and p1 as shown here:
```
ALTER TABLE t1 DROP PARTITION p0, p1;
```
Note that DROP PARTITION does not work with tables that use the NDB Cluster storage engine. See Section 18.3.1, “Management of RANGE and LIST Partitions”, and Section 17.11, “Known Limitations of MySQL Cluster”.

ADD PARTITION and DROP PARTITION do not currently support IF [NOT] EXISTS. It is also not possible to rename a partition or a partitioned table. Instead, if you wish to rename a partition, you must drop and re-create the partition; if you wish to rename a partitioned table, you must instead drop all partitions, rename the table, and then add back the partitions that were dropped.

COALESCE PARTITION can be used with a table that is partitioned by HASH or KEY to reduce the number of partitions by number. Suppose that you have created table t2 using the following definition:
```
CREATE TABLE t2 (
    name VARCHAR (30),
    started DATE
)
PARTITION BY HASH( YEAR(started) )
PARTITIONS 6;
```
You can reduce the number of partitions used by t2 from 6 to 4 using the following statement:
```
ALTER TABLE t2 COALESCE PARTITION 2;
```
The data contained in the last number partitions will be merged into the remaining partitions. In this case, partitions 4 and 5 will be merged into the first 4 partitions (the partitions numbered 0, 1, 2, and 3.

To change some but not all the partitions used by a partitioned table, you can use REORGANIZE PARTITION. This statement can be used in several ways:
- To merge a set of partitions into a single partition. This can be done by naming several partitions in the partition_names list and supplying a single definition for partition_definition.
- To split an existing partition into several partitions. You can accomplish this by naming a single partition for partition_names and providing multiple partition_definitions.
- To change the ranges for a subset of partitions defined using VALUES LESS THAN or the value lists for a subset of partitions defined using VALUES IN.
Note: For partitions that have not been explicitly named, MySQL automatically provides the default names p0, p1, p2, and so on. As of MySQL 5.1.7, the same is true with regard to subpartitions.

For more detailed information about and examples of ALTER TABLE ... REORGANIZE PARTITION statements, see Section 18.3, “Partition Management”.
Several additional clauses provide partition maintenance and repair functionality analogous to that implemented for non-partitioned tables by statements such as CHECK TABLE and REPAIR TABLE (which are not supported for partitioned tables). These include ANALYZE PARTITION, CHECK PARTITION, OPTIMIZE PARTITION, REBUILD PARTITION, and REPAIR PARTITION. Each of these options takes a partition_names clause consisting of one or more names of partitions, separated by commas. The partitions must already exist in the table to be altered. For more information, and for examples of these, see Section 18.3.3, “Maintenance of Partitions”.
REMOVE PARTITIONING was introduced in MySQL 5.1.8 for the purpose of removing a table's partitioning without otherwise affecting the table or its data. (Previously. this was done using the ENGINE option.) This option can be combined with other ALTER TABLE options such as those used to add, drop, or rename drop columns or indexes.

In MySQL 5.1.7 and earlier, using the ENGINE option with ALTER TABLE caused any partitioning that a table might have had to be removed. Beginning with MySQL 5.1.8, this option merely changes the storage engine used by the table and no longer affects partitioning in any way.

With the mysql_info() C API function, you can find out how many rows were copied, and (when IGNORE is used) how many rows were deleted due to duplication of unique key values. See Section 26.2.3.34, “mysql_info()”.

Here are some examples that show uses of ALTER TABLE. Begin with a table t1 that is created as shown here:

CREATE TABLE t1 (a INTEGER,b CHAR(10));

To rename the table from t1 to t2:

ALTER TABLE t1 RENAME t2;

To change column a from INTEGER to TINYINT NOT NULL (leaving the name the same), and to change column b from CHAR(10) to CHAR(20) as well as renaming it from b to c:

ALTER TABLE t2 MODIFY a TINYINT NOT NULL, CHANGE b c CHAR(20);

To add a new TIMESTAMP column named d:

ALTER TABLE t2 ADD d TIMESTAMP;

To add indexes on column d and on column a:

ALTER TABLE t2 ADD INDEX (d), ADD INDEX (a);

To remove column c:

ALTER TABLE t2 DROP COLUMN c;

To add a new AUTO_INCREMENT integer column named c:

ALTER TABLE t2 ADD c INT UNSIGNED NOT NULL AUTO_INCREMENT,
  ADD PRIMARY KEY (c);

Note that we indexed c (as a PRIMARY KEY), because AUTO_INCREMENT columns must be indexed, and also that we declare c as NOT NULL, because primary key columns cannot be NULL.

When you add an AUTO_INCREMENT column, column values are filled in with sequence numbers for you automatically. For MyISAM tables, you can set the first sequence number by executing SET INSERT_ID=value before ALTER TABLE or by using the AUTO_INCREMENT=value table option. See Section 13.5.3, “SET Syntax”.

You can use the ALTER TABLE ... AUTO_INCREMENT=value table option for InnoDB tables to set the sequence number for new rows if the value is greater than the maximum value in the AUTO_INCREMENT column. If the value is less than the current maximum value in the column, no error message is given and the current sequence value is not changed.

With MyISAM tables, if you do not change the AUTO_INCREMENT column, the sequence number is not affected. If you drop an AUTO_INCREMENT column and then add another AUTO_INCREMENT column, the numbers are resequenced beginning with 1.

When replication is used, adding an AUTO_INCREMENT column to a table might not produce the same ordering of the rows on the slave and the master. This occurs because the order in which the rows are numbered depends on the specific storage engine used for the table and the order in which the rows were inserted. If it is important to have the same order on the master and slave, the rows must be ordered before assigning an AUTO_INCREMENT number. Assuming that you want to add an AUTO_INCREMENT column to the table t1, the following statements produce a new table t2 identical to t1 but with an AUTO_INCREMENT column:

CREATE TABLE t2 (id INT AUTO_INCREMENT PRIMARY KEY) 
SELECT * FROM t1 ORDER BY col1, col2;

This assumes that the table t1 has columns col1 and col2.

This set of statements will also produce a new table t2 identical to t1, with the addition of an AUTO_INCREMENT column:

CREATE TABLE t2 LIKE t1;
ALTER TABLE T2 ADD id INT AUTO_INCREMENT PRIMARY KEY;
INSERT INTO t2 SELECT * FROM t1 ORDER BY col1, col2;

Important: To guarantee the same ordering on both master and slave, all columns of t1 must be referenced in the ORDER BY clause.

Regardless of the method used to create and populate the copy having the AUTO_INCREMENT column, the final step is to drop the original table and then rename the copy:

DROP t1;
ALTER TABLE t2 RENAME t1;

User Comments

Posted by

可以不需要修改my.ini。在建立数据库的时候，对库和表的字符集设置不太重要，但是对文本类型的字段最好都设置为GBK字符集。
对于已有的数据库可以用以下方法转换字段编码：

ALTER TABLE t MODIFY hoverfly BINARY(100);

ALTER TABLE t MODIFY hoverfly CHAR(100) CHARACTER SET gbk;

注意用此方法如果不修改程序，会发现查询出的结果都是乱码，可以通过在my.ini的[MySQLd]段内加default-character-set=gbk来纠正。但是这样你会发现那些没有转换编码的文本字段里的中文都是乱码。
其实有更简单的办法，在进行查询前，只要执行SET character_set_results = NULL就可以。而且这是不管是转换了的还是没转换的字段都不会出现乱码。而转换了的字段可以正常的对中文进行排序。

Posted by Tom S on December 18 2002 4:27pm