A2-03-11.DDL-How to Add Columns to a Table Using MySQL ADD COLUMN Statement
转载自：http://www.mysqltutorial.org/mysql-add-column/
How to Add Columns to a Table Using MySQL ADD COLUMN Statement
Summary: in this tutorial, we will show you how to add a column to a table using MySQL ADD COLUMN statement.
Introduction to MySQL ADD COLUMN statement
To add a new column to an existing table, you use the ALTER TABLE ADD COLUMN statement as follows:
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ALTER TABLE table
ADD [COLUMN] column_name column_definition [FIRST|AFTER existing_column];
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Let’s examine the statement in more detail.
- First, you specify the table name after the
ALTER TABLEclause. - Second, you put the new column and its definition after the
ADD COLUMNclause. Note thatCOLUMNkeyword is optional so you can omit it. - Third, MySQL allows you to add the new column as the first column of the table by specifying the
FIRSTkeyword. It also allows you to add the new column after an existing column using theAFTER existing_columnclause. If you don’t explicitly specify the position of the new column, MySQL will add it as the last column.
To add two or more columns to a table at the same time, you use the following syntax:
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ALTER TABLE table
ADD [COLUMN] column_name_1 column_1_definition [FIRST|AFTER existing_column],
ADD [COLUMN] column_name_2 column_2_definition [FIRST|AFTER existing_column],
...;
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Let’s take a look some examples of adding a new column to an existing table.
MySQL ADD COLUMN examples
First, we create a table named vendors for the demonstration purpose using the following statement:
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CREATE TABLE IF NOT EXISTS vendors (
id INT AUTO_INCREMENT PRIMARY KEY,
name VARCHAR(255)
);
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Second, we add a new column named phone to the vendors table. Because we specify the position of the phone column explicitly after the name column, MySQL will obey this.
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ALTER TABLE vendors
ADD COLUMN phone VARCHAR(15) AFTER name;
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Third, we add a new column named vendor_group to the vendors table. At this time, we don’t specify the new column’s position so MySQL adds the vendor_group column as the last column of the vendors table.
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ALTER TABLE vendors
ADD COLUMN vendor_group INT NOT NULL;
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Let’s insert some rows into the vendors table.
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INSERT INTO vendors(name,phone,vendor_group)
VALUES('IBM','(408)-298-2987',1);
INSERT INTO vendors(name,phone,vendor_group)
VALUES('Microsoft','(408)-298-2988',1);
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We can query the data of the vendors table to see the changes.
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SELECT
id, name, phone,vendor_group
FROM
vendors;
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Fourth, add two more columns email and hourly_rate to the vendors table at the same time.
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ALTER TABLE vendors
ADD COLUMN email VARCHAR(100) NOT NULL,
ADD COLUMN hourly_rate decimal(10,2) NOT NULL;
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Note that both email and hourly_rate columns are assigned to NOT NULL values However, the vendorstable already has data. In such cases, MySQL will use default values for those new columns.
Let’s check the data in the vendors table.
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SELECT
id, name, phone, vendor_group, email, hourly_rate
FROM
vendors;
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The email column is populated with blank values, not the NULL values. And the hourly_rate column is populated with 0.00 values.
If you accidentally add a column that already exists in the table, MySQL will issue an error. For example, if you execute the following statement:
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ALTER TABLE vendors
ADD COLUMN vendor_group INT NOT NULL;
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MySQL issued an error message:
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Error Code: 1060. Duplicate column name 'vendor_group'
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For the table with a few columns, it is easy to see which columns are already there. However, with a big table with hundred of columns, it is more difficult.
In some situations, you want to check whether a column already exists in a table before adding it. However, there is no statement like ADD COLUMN IF NOT EXISTS available. Fortunately, you can get this information from the columns table of the information_schema database as the following query:
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SELECT
IF(count(*) = 1, 'Exist','Not Exist') AS result
FROM
information_schema.columns
WHERE
table_schema = 'classicmodels'
AND table_name = 'vendors'
AND column_name = 'phone';
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In the WHERE clause, we passed three arguments: table schema or database, table name, and column name. We used IF function to return whether the column exists or not.
In this tutorial, you have learned how to add one or more columns to a table using MySQL ADD COLUMN statement.
