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Modifies a table definition by modifying, adding, or dropping columns and constraints.



ALTER TABLE table_name
    ALTER COLUMN column_name 
   {
    type_name[({precision[.scale]})][NULL|NOT NULL]
   {DROP DEFAULT 
   | SET DEFAULT constant_expression 
   | IDENTITY [ ( seed , increment ) ]
   } 
| ADD 
   { < column_definition > | < table_constraint > } [ ,...n ] 
| DROP 
   { [ CONSTRAINT ] constraint_name 
   | COLUMN column }
] }
< column_definition > ::= 
   { column_name data_type } 
   [ [ DEFAULT constant_expression ] 
      | IDENTITY [ ( seed , increment ) ] 
   ] 
   [ROWGUIDCOL]
   [ < column_constraint > ] [ ...n ] ]
< column_constraint > ::= 
   [ NULL | NOT NULL ] 
   [ CONSTRAINT constraint_name ] 
   { 
      | { PRIMARY KEY | UNIQUE } 
      | REFERENCES ref_table [ (ref_column) ] 
      [ ON DELETE { CASCADE | NO ACTION | SET DEFAULT |SET NULL } ] 
      [ ON UPDATE { CASCADE | NO ACTION | SET DEFAULT |SET NULL } ]
   }
< table_constraint > ::= 
   [ CONSTRAINT constraint_name ] 
   { [ { PRIMARY KEY | UNIQUE } 
      { ( column [ ,...n ] ) } 
      | FOREIGN KEY 
        ( column [ ,...n ] )
        REFERENCES ref_table [ (ref_column [ ,...n ] ) ] 
      [ ON DELETE { CASCADE | NO ACTION | SET DEFAULT |SET NULL } ] 
      [ ON UPDATE { CASCADE | NO ACTION | SET DEFAULT |SET NULL } ] 
   }

table_name

The name of the table to be modified.

Table names must comply with the rules for identifiers. The table_name must be unique within the database. The table_name can contain a maximum of 128 characters.

ALTER COLUMN

Specifies that the particular column is to be changed or modified.

column_name

The name of a column in the table. Column names must comply with the rules for identifiers and must be unique in the table.

data_type

Specifies the column data type. For more information about data types, see Data Types.

DEFAULT

Specifies the value provided for the column when a value is not explicitly supplied during an insert action. DEFAULT definitions can be applied to any column except those defined by the IDENTITY property. DEFAULT definitions are removed when the table is dropped. Only a constant value, such as a character string or a date function, can be used as a default.

IDENTITY

Indicates that the new column is an identity column. When a new row is added to the table, Microsoft SQL Server Compact 3.5 provides an incremental value for the column. Identity columns are typically used in conjunction with PRIMARY KEY constraints to serve as the unique row identifier for the table. The IDENTITY property can be assigned only to int columns. Only one identity column can be created per table. Bound defaults and DEFAULT constraints cannot be used with an identity column. You must specify both the seed and increment, or neither. If neither is specified, the default is (1,1).

seed

The value used for the first row that is loaded into the table.

increment

The incremental value added to the identity value of the previous row that is loaded.

ADD

Specifies that one or more column definitions or table constraints are added.

DROP { [CONSTRAINT] constraint_name| COLUMN column}

Specifies that constraint_name or column_name is to be removed from the table.

ROWGUIDCOL

Indicates that the new column is a row global unique identifier column. Only one uniqueidentifier column per table can be designated as the ROWGUIDCOL column. The ROWGUIDCOL property can be assigned only to a uniqueidentifier column.

ROWGUIDCOL automatically generates values for new rows inserted into the table.

NULL | NOT NULL

Keywords that specify whether null values are permitted in the column. NULL is not strictly a constraint, but can be specified in the same manner as NOT NULL.

CONSTRAINT

An optional keyword indicating the beginning of a definition for a PRIMARY KEY, UNIQUE, or FOREIGN KEY constraint. Constraints are special properties that enforce data integrity and create special types of indexes for the table and its columns.

constraint_name

The name of a constraint. The constraint_name is optional and must be unique within a database. If the constraint_name is not specified, SQL Server Compact 3.5 generates a constraint name.

PRIMARY KEY

A constraint that enforces entity integrity for a particular column or columns by using a unique index. Only one PRIMARY KEY constraint can be created per table.

UNIQUE

A constraint that provides entity integrity for a particular column or columns using a unique index. Columns in a UNIQUE constraint may be NULL, but only one NULL value is allowed per column. A table can have multiple UNIQUE constraints.

SQL Server Compact 3.5 can use indexes to enforce PRIMARY KEY and UNIQUE constraints. We recommend that you do not rely on this behavior nor try to modify any indexes that are created as part of a constraint.

FOREIGN KEY...REFERENCES

A constraint that provides referential integrity for the data in the column. FOREIGN KEY constraints require that each value in the column exists in the specified column in the referenced table.

ref_table

The name of the table referenced by the FOREIGN KEY constraint.

( ref_column [ ,... n] )

A column or list of columns from the table that is referenced by the FOREIGN KEY constraint.

ON DELETE {CASCADE | NO ACTION |SET DEFAULT | SET NULL}

Specifies what action happens to a row in the table that is created when that row has a referential relationship and the referenced row is deleted from the parent table. The default is NO ACTION. See the "Remarks" section later in this topic for more information.

ON UPDATE {CASCADE | NO ACTION | SET DEFAULT | SET NULL}

Specifies what action happens to a row in the table that is created when that row has a referential relationship, and the referenced row is updated in the parent table. The default is NO ACTION. See the "Remarks" section later in this topic for more information.

column

A column or list of columns, in parentheses, used in table constraints to indicate the columns used in the definition of the constraint.

n

A placeholder indicating that the preceding item can be repeated n number of times.


For ON DELETE or ON UPDATE, if the CASCADE option is specified, the row is updated in the referencing table if the corresponding referenced row is updated in the parent table. If NO ACTION is specified, SQL Server Compact 3.5 returns an error, and the update action on the referenced row in the parent table is rolled back.

For example, you might have two tables, A and B, in a database. Table A has a referential relationship with table B: the A.ItemID foreign key references the B.ItemID primary key.

If an UPDATE statement is executed on a row in table B and an ON UPDATE CASCADE action is specified for A.ItemID, SQL Server Compact 3.5 checks for one or more dependent rows in table A. If any exist, the dependent rows in table A are updated, as is the row referenced in table B.

Alternatively, if NO ACTION is specified, SQL Server Compact 3.5 returns an error and rolls back the update action on the referenced row in table B when there is at least one row in table A that references it.

A. Changing the seed and increment values on the identity column

The following example changes the seed and increment values on the identity column.

CREATE TABLE MyCustomers (CustID INTEGER IDENTITY (100,1) PRIMARY KEY, CompanyName NvarChar (50));

INSERT INTO MyCustomers (CompanyName) VALUES ('A. Datum Corporation');

ALTER TABLE MyCustomers ALTER COLUMN CustId IDENTITY (200, 2);

B. Adding a default value to a column

The following example modifies the CompanyName column so that it has a default value.

ALTER TABLE MyCustomers ALTER COLUMN CompanyName SET DEFAULT 'A. Datum Corporation'

C. Dropping a default value from the column

The following example modifies the CompanyName column so that it does not have a default value.

ALTER TABLE MyCustomers ALTER COLUMN CompanyName DROP DEFAULT
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