学步园

Pseudocolumns

A pseudocolumn behaves like a table column, but is not actually stored in the table. You can select from pseudocolumns, but you cannot insert, update, or delete their values. This section describes these pseudocolumns:

CURRVAL and NEXTVAL

A sequence is a schema object that can generate unique sequential values. These values are often used for primary and unique keys. You can refer to sequence values in SQL statements with these pseudocolumns:

• CURRVAL: returns the current value of a sequence
• NEXTVAL: increments the sequence and returns the next value

You must qualify CURRVAL and NEXTVAL with the name of the sequence:

sequence.CURRVAL

sequence.NEXTVAL

To refer to the current or next value of a sequence in the schema of another user, you must have been granted either SELECT object privilege on the sequence or SELECT ANY SEQUENCE system privilege, and you must qualify the sequence with the schema containing it:

schema.sequence.CURRVAL

schema.sequence.NEXTVAL

To refer to the value of a sequence on a remote database, you must qualify the sequence with a complete or partial name of a database link:

schema.sequence.CURRVAL@dblink

schema.sequence.NEXTVAL@dblink

ROWID

For each row in the database, the ROWID pseudocolumn returns a row's address. Oracle9i rowid values contain information necessary to locate a row:

• The data object number of the object
• Which data block in the datafile
• Which row in the data block (first row is 0)
• Which datafile (first file is 1). The file number is relative to the tablespace.

Usually, a rowid value uniquely identifies a row in the database. However, rows in different tables that are stored together in the same cluster can have the same rowid.

Values of the ROWID pseudocolumn have the datatype ROWID or UROWID.

Rowid values have several important uses:

• They are the fastest way to access a single row.
• They can show you how a table's rows are stored.
• They are unique identifiers for rows in a table.

You should not use ROWID as a table's primary key. If you delete and reinsert a row with the Import and Export utilities, for example, then its rowid may change. If you delete a row, then Oracle may reassign its rowid to a new row inserted later.

Although you can use the ROWID pseudocolumn in the SELECT and WHERE clause of a query, these pseudocolumn values are not actually stored in the database. You cannot insert, update, or delete a value of the ROWID pseudocolumn.

Example

This statement selects the address of all rows that contain data for employees in department 20:

SELECT ROWID, last_name  

   FROM employees

   WHERE department_id = 20;

ROWNUM

For each row returned by a query, the ROWNUM pseudocolumn returns a number indicating the order in which Oracle selects the row from a table or set of joined rows. The first row selected has a ROWNUM of 1, the second has 2, and so on.

You can use ROWNUM to limit the number of rows returned by a query, as in this example:

SELECT * FROM employees WHERE ROWNUM < 10;

If an ORDER BY clause follows ROWNUM in the same query, then the rows will be reordered by the ORDER BY clause. The results can vary depending on the way the rows are accessed. For example, if the ORDER BY clause causes Oracle to use an index to access the data, then Oracle may retrieve the rows in a different order than without the index. Therefore, the following statement will not have the same effect as the preceding example:

SELECT * FROM employees WHERE ROWNUM < 11 ORDER BY last_name;

If you embed the ORDER BY clause in a subquery and place the ROWNUM condition in the top-level query, then you can force the ROWNUM condition to be applied after the ordering of the rows. For example, the following query returns the 10 smallest employee numbers. This is sometimes referred to as a "top-N query":

SELECT * FROM

   (SELECT * FROM employees ORDER BY employee_id)

   WHERE ROWNUM < 11;

In the preceding example, the ROWNUM values are those of the top-level SELECT statement, so they are generated after the rows have already been ordered by employee_id in the subquery.

Conditions testing for ROWNUM values greater than a positive integer are always false. For example, this query returns no rows:

SELECT * FROM employees

    WHERE ROWNUM > 1;

The first row fetched is assigned a ROWNUM of 1 and makes the condition false. The second row to be fetched is now the first row and is also assigned a ROWNUM of 1 and makes the condition false. All rows subsequently fail to satisfy the condition, so no rows are returned.

You can also use ROWNUM to assign unique values to each row of a table, as in this example:

UPDATE my_table

    SET column1 = ROWNUM;