ALTER TABLE statement

The ALTER TABLE statement allows you to:

• add a column to a table
• add a constraint to a table
• drop an existing constraint from a table
• increase the width of a VARCHAR, CHAR VARYING, and CHARACTER VARYING column
• add a default value for an existing column in a table
• override row-level locking for the table (or drop the override)

Format

ALTER TABLE table-Name
{
    ADD COLUMN column-definition |
    ADD CONSTRAINT clause |
    DROP { PRIMARY KEY | FOREIGN KEY constraint-name | UNIQUE 
     constraint-name | CHECK constraint-name | CONSTRAINT constraint-name }
    ALTER column-alteration |
    LOCKSIZE { ROW | TABLE }
}

column-definition:  

Simple-column-Name DataType
[ Column-Level-Constraint ]*
[ [ WITH ] DEFAULT {ConstantExpression | NULL } ]

column-alteration:  

column-Name SET DATA TYPE VARCHAR(integer) |
column-name SET INCREMENT BY integer-constant 

In the column-alteration, SET INCREMENT BY integer-constant, specifies the interval between consecutive values of the identity column. The next value to be generated for the identity column will be determined from the last assigned value with the increment applied. The column must already be defined with the IDENTITY attribute.

ALTER TABLE does not affect any view that references the table being altered. This includes views that have an "*" in their SELECT list. You must drop and re-create those views if you wish them to return the new columns.

The modifications you can make using ALTER TABLE are explained in the following sections:

• Adding Columns
• Adding Constraints
• Dropping Constraints
• Modifying Columns
• Setting Defaults
• Changing the Lock Granularity for the Table

Adding Columns

The syntax for the column-definition for a new column is the same as for a column in a CREATE TABLE statement. This means that a column constraint can be placed on the new column within the ALTER TABLE ADD COLUMN statement. However, a column with a NOT NULL constraint can be added to an existing table only if the table is empty or if you give a default value; otherwise, an exception is thrown when the ALTER TABLE statement is executed.

Just as in CREATE TABLE, if the column definition includes a unique or primary key constraint, the column cannot contain null values, so the NOT NULL attribute must also be specified (SQLSTATE 42831).

See Adding Constraints for the other limitations.

Note:

If a table has an UPDATE trigger without an explicit column list, adding a column to that table in effect adds that column to the implicit update column list upon which the trigger is defined, and all references to transition variables are invalidated so that they pick up the new column.

Adding Constraints

ALTER TABLE ADD CONSTRAINT adds a table-level constraint to an existing table. Any supported table-level constraint type can be added via ALTER TABLE. The following limitations exist on adding a constraint to an existing table:

• When adding a foreign key or check constraint to an existing table, Derby checks the table to make sure existing rows satisfy the constraint. If any row is invalid, Derby throws a statement exception and the constraint is not added.
• All columns included in a primary key must contain non null data and be unique.

  ALTER TABLE ADD UNIQUE or PRIMARY KEY provide a shorthand method of defining a primary key composed of a single column. If PRIMARY KEY is specified in the definition of column C, the effect is the same as if the PRIMARY KEY(C) clause were specified as a separate clause. The column cannot contain null values, so the NOT NULL attribute must also be specified.

For information on the syntax of constraints, see CONSTRAINT clause. Use the syntax for table-level constraint when adding a constraint with the ADD TABLE ADD CONSTRAINT syntax.

Dropping Constraints

ALTER TABLE DROP CONSTRAINT drops a constraint on an existing table. To drop an unnamed constraint, you must specify the generated constraint name stored in SYS.SYSCONSTRAINTS as a delimited identifier.

Dropping a primary key, unique, or foreign key constraint drops the physical index that enforces the constraint (also known as a backing index).

Modifying Columns

The column-alteration allows you to alter the named column in the following ways:

• Increasing the length of an existing VARCHAR column. CHARACTER VARYING or CHAR VARYING can be used as synonyms for the VARCHAR keyword.

  To increase the width of a column of these types, specify the data type and new size after the column name.

  You are not allowed to decrease the width or to change the data type. You are not allowed to increase the width of a column that is part of a primary or unique key referenced by a foreign key constraint or that is part of a foreign key constraint.

• Specifying the interval between consecutive values of the identity column.
• To set an interval between consecutive values of the identity column, specify the integer-constant.
• You must previously define the column with the IDENTITY attribute (SQLSTATE 42837).

Setting Defaults

You can specify a default value for a new column or add a default value to an existing column (see Modifying Columns). A default value is the value that is inserted into a column if no other value is specified. If not explicitly specified, the default value of a column is NULL. If you add a default to a new column, existing rows in the table gain the default value in the new column. If you add a default to an existing column, existing rows in the table do not gain the default value in the new column.

You can set the interval between consecutive values of the identity column to an existing column in a table. If there are existing rows in the table, the values in the column for which the SET INCREMENT default was added do not change. Note that this means that values in the column are not guaranteed to be unique (use a unique or primary key constraint to guarantee uniqueness).

For more information about defaults, see CREATE TABLE statement.

Changing the Lock Granularity for the Table

The LOCKSIZE clause allows you to override row-level locking for the specific table, if your system uses the default setting of row-level locking. (If your system is set for table-level locking, you cannot change the locking granularity to row-level locking, although Derby allows you to use the LOCKSIZE clause in such a situation without throwing an exception.) To override row-level locking for the specific table, set locking for the table to TABLE. If you created the table with table-level locking granularity, you can change locking back to ROW with the LOCKSIZE clause in the ALTER TABLE. For information about why this is sometimes useful, see "About the System's Selection of Lock Granularity" in Chapter 4 of Tuning Derby.

Examples

-- Add a new column with a column-level constraint
-- to an existing table
-- An exception will be thrown if the table
-- contains any rows
-- since the newcol will be initialized to NULL
-- in all existing rows in the table 
ALTER TABLE CITIES ADD COLUMN REGION VARCHAR(26)
CONSTRAINT NEW_CONSTRAINT CHECK (REGION IS NOT NULL);
 
 -- Add a new unique constraint to an existing table
-- An exception will be thrown if duplicate keys are found 
ALTER TABLE SAMP.DEPARTMENT
ADD CONSTRAINT NEW_UNIQUE UNIQUE (DEPTNO);
 
 -- add a new foreign key constraint to the
-- Cities table. Each row in Cities is checked
-- to make sure it satisfied the constraints.
-- if any rows don't satisfy the constraint, the
-- constraint is not added 
ALTER TABLE CITIES ADD CONSTRAINT COUNTRY_FK
Foreign Key (COUNTRY) REFERENCES COUNTRIES (COUNTRY);
 
 -- Add a primary key constraint to a table
-- First, create a new table 
CREATE TABLE ACTIVITIES (CITY_ID INT NOT NULL,
SEASON CHAR(2), ACTIVITY VARCHAR(32) NOT NULL);
 -- You will not be able to add this constraint if the
-- columns you are including in the primary key have
-- null data or duplicate values. 
ALTER TABLE Activities ADD PRIMARY KEY (city_id, activity);
 
 -- Drop a primary key constraint from the CITIES table
 ALTER TABLE Cities DROP CONSTRAINT Cities_PK;
 -- Drop a foreign key constraint from the CITIES table 
ALTER TABLE Cities DROP CONSTRAINT COUNTRIES_FK;
 -- add a DEPTNO column with a default value of 1 
ALTER TABLE SAMP.EMP_ACT ADD COLUMN DEPTNO INT DEFAULT 1;
 -- increase the width of a VARCHAR column 
ALTER TABLE SAMP.EMP_PHOTO ALTER PHOTO_FORMAT SET DATA TYPE VARCHAR(30);
 -- change the lock granularity of a table 
ALTER TABLE SAMP.SALES LOCKSIZE TABLE;

Results

An ALTER TABLE statement causes all statements that are dependent on the table being altered to be recompiled before their next execution. ALTER TABLE is not allowed if there are any open cursors that reference the table being altered.