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Creating Tables and Views
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The results will show the identity value that was inserted in the myID column for each row as follows:
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myID -----1 2 3 firstName ----------Ben Paul George lastName ----------Franklin Revere Washington
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The myID column was automatically populated with the next value in the sequence. You will get an error if you try to fill in a value for an identity column. If you need to know the value of the identity column of the last row inserted, you can use the built-in function @@IDENTITY. After the three insert statements in the previous example, you can add the following statement to see the identity value that was last inserted.
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This will return a 3, as that was the last identity value inserted into myTable. SQL Server does not guarantee sequential gap-free values in identity columns. If records are deleted, SQL Server will not back fill the missing values. By the same token, if you delete all the records from a table, it will not reset the identity. The next identity number will be the next number in sequence. After the following code is executed in the myTable example, you will see that the identity column sequence picks up where it last left off. First we delete a row from the table with this DELETE statement:
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DELETE FROM myTable WHERE lastName = Revere
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Then we insert a new row:
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INSERT INTO myTable(firstName, lastName) VALUES ( Abe , Lincoln )
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Then we select all rows from the table to see the identity values now:
The rows returned show that the next identity value of 4 was used for the inserted row and the previously used identity value of 2 is now deleted from the table and will not be used again. Here are the results:
myID -----1 4 3 firstName ---------Ben Abe George lastName ----------Franklin Lincoln Washington
Part III
Microsoft SQL Server 2005 Administration
There is a way to set the value of an identity column explicitly when you are inserting a record. This is accomplished by first setting the IDENTITY_INSERT property to ON, then inserting the row:
SET IDENTITY_INSERT myTable ON INSERT myTable (myID, firstName, lastName) Values(2, John , Hancock ) SET IDENTITY_INSERT myTable OFF
The identity value of 2 was allowed to be explicitly inserted into myTable. Now select all rows to see the values:
The new results for the table are as follows:
myID -----1 4 2 3 firstName ---------Ben Abe John George lastName ----------Washington Lincoln Hancock Washington
Only one table in a session can have the IDENTITY_INSERT property set to ON. It is a good practice to turn it on to use it and then immediately turn it back off.
Creating, Modifying, and Dropping Tables
When you create a table, you must specify the table name, column names, and column data types. Column names must be unique to a table, but the same column name can be used in different tables within the same database. The column name can be omitted for columns that are created with a timestamp data type, in which case the column name will default to timestamp.
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You should adopt standards when naming your tables and columns. Making good use of uppercase and lowercase letters and using underscores to separate words will help make the table and columns easier to read and understand. Try to keep the names short
Creating Tables and Views
but still long enough to be descriptive. If you use the same column in different tables, try to use the same name. This consistency helps avoid confusion when creating and using queries. Below is the basic T-SQL syntax for creating a table. Not all of the arguments for creating a table are listed here. Refer to SQL Server Books Online for the complete CREATE TABLE syntax:
CREATE TABLE [ database_name . [ schema_name ] . | schema_name . ] table_name ( column_name <data_type> [ NULL | NOT NULL ] [ DEFAULT constant_expression ] [ IDENTITY [ ( seed ,increment ) ] [ ,...n ] )
Below are descriptions of the CREATE TABLE arguments listed in the above syntax:
database_name Optional. Name of the database in which the table will be created. Defaults to the current database. schema_name Optional. Name of the schema to which the new table belongs. Defaults to dbo. table_name Required. Name of the new table. column_name Required. Name of the column. data_type Required. System or user-defined data type including arguments (precision, scale, max, and so on) if applicable. nullability Optional. Indicates whether null values are allowed in the column. Simply state NULL or NOT NULL. NOT NULL is the SQL Server default, but the server defaults can be changed. It is best to specify the nullability of a column when creating a table. DEFAULT Optional. The value you want to be used for the column for any row that is inserted without explicitly supplying a value for that particular column. IDENTITY Optional. Indicates that the column is an identity column. The (seed, increment) values will default to (1, 1) if not specified.
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