ORACLE SQL總結四：視圖、序列、索引和同義詞

一、序列

1.1 序列的定義和做用

Use the CREATE SEQUENCE statement to create a sequence, which is a database object from which multiple users may generate unique integers. You can use sequences to automatically generate primary key values.

When a sequence number is generated, the sequence is incremented, independent of the transaction committing or rolling back. If two users concurrently increment the same sequence, then the sequence numbers each user acquires may have gaps, because sequence numbers are being generated by the other user. One user can never acquire the sequence number generated by another user. After a sequence value is generated by one user, that user can continue to access that value regardless of whether the sequence is incremented by another user.

Sequence numbers are generated independently of tables, so the same sequence can be used for one or for multiple tables. It is possible that individual sequence numbers will appear to be skipped, because they were generated and used in a transaction that ultimately rolled back. Additionally, a single user may not realize that other users are drawing from the same sequence.

After a sequence is created, you can access its values in SQL statements with the CURRVAL pseudocolumn, which returns the current value of the sequence, or the NEXTVAL pseudocolumn, which increments the sequence and returns the new value

1.2 在哪裏使用序列和哪裏不能使用 Where to Use Sequence Values

You can use CURRVAL and NEXTVAL in the following locations:

• The select list of a SELECT statement that is not contained in a subquery, materialized view, or view

• The select list of a subquery in an INSERT statement

• The VALUES clause of an INSERT statement

• The SET clause of an UPDATE statement

Restrictions on Sequence Values You cannot use CURRVAL and NEXTVAL in the following constructs:

• A subquery in a DELETE, SELECT, or UPDATE statement

• A query of a view or of a materialized view

• A SELECT statement with the DISTINCT operator

• A SELECT statement with a GROUP BY clause or ORDER BY clause

• A SELECT statement that is combined with another SELECT statement with the UNION, INTERSECT, or MINUS set operator

• The WHERE clause of a SELECT statement

• The DEFAULT value of a column in a CREATE TABLE or ALTER TABLE statement

• The condition of a CHECK constraint

Within a single SQL statement that uses CURRVAL or NEXTVAL, all referenced LONG columns, updated tables, and locked tables must be located on the same database.

1.3 何時序列增加 Within a single SQL statement containing a reference to NEXTVAL, Oracle increments the sequence once:

• For each row returned by the outer query block of a SELECT statement. Such a query block can appear in the following places:

  • A top-level SELECT statement

  • An INSERT ... SELECT statement (either single-table or multitable). For a multitable insert, the reference to NEXTVAL must appear in the VALUES clause, and the sequence is updated once for each row returned by the subquery, even though NEXTVAL may be referenced in multiple branches of the multitable insert.

  • A CREATE TABLE ... AS SELECT statement

  • A CREATE MATERIALIZED VIEW ... AS SELECT statement

• For each row updated in an UPDATE statement

• For each INSERT statement containing a VALUES clause

• For each INSERT ... [ALL | FIRST] statement (multitable insert). A multitable insert is considered a single SQL statement. Therefore, a reference to the NEXTVAL of a sequence will increase the sequence only once for each input record coming from the SELECT portion of the statement. If NEXTVAL is specified more than once in any part of the INSERT ... [ALL |FIRST ] statement, then the value will be the same for all insert branches, regardless of how often a given record might be inserted.

• For each row merged by a MERGE statement. The reference to NEXTVAL can appear in the merge_insert_clause or the merge_update_clause or both. The NEXTVALUE value is incremented for each row updated and for each row inserted, even if the sequence number is not actually used in the update or insert operation. If NEXTVAL is specified more than once in any of these locations, then the sequence is incremented once for each row and returns the same value for all occurrences of NEXTVAL for that row.

• For each input row in a multitable INSERT ALL statement. NEXTVAL is incremented once for each row returned by the subquery, regardless of how many occurrences of theinsert_into_clause map to each row.

1.4 序列的一些特殊狀況

If any of these locations contains more than one reference to NEXTVAL, then Oracle increments the sequence once and returns the same value for all occurrences of NEXTVAL.

If any of these locations contains references to both CURRVAL and NEXTVAL, then Oracle increments the sequence and returns the same value for both CURRVAL and NEXTVAL.

二、視圖 view

2.1 視圖的一些特殊狀況

Rows cannot be deleted through a view if the view definition contains the DISTINCT keyword.

三、約束狀態 Constraint State

3.1 DEFERRABLE

DEFERRABLE Clause The DEFERRABLE and NOT DEFERRABLE parameters indicate whether or not, in subsequent transactions, constraint checking can be deferred until the end of the transaction using the SET CONSTRAINT(S) statement. If you omit this clause, then the default is NOT DEFERRABLE.

• Specify NOT DEFERRABLE to indicate that in subsequent transactions you cannot use the SET CONSTRAINT[S] clause to defer checking of this constraint until the transaction is committed. The checking of a NOT DEFERRABLE constraint can never be deferred to the end of the transaction.

  If you declare a new constraint NOT DEFERRABLE, then it must be valid at the time the CREATE TABLE or ALTER TABLE statement is committed or the statement will fail.

• Specify DEFERRABLE to indicate that in subsequent transactions you can use the SET CONSTRAINT[S] clause to defer checking of this constraint until a COMMIT statement is submitted. If the constraint check fails, then the database returns an error and the transaction is not committed. This setting in effect lets you disable the constraint temporarily while making changes to the database that might violate the constraint until all the changes are complete.

You cannot alter the deferrability of a constraint. Whether you specify either of these parameters, or make the constraint NOT DEFERRABLE implicitly by specifying neither of them, you cannot specify this clause in an ALTER TABLE statement. You must drop the constraint and re-create it.

指定爲DEFERRABLE才能使用INITIALLY clause,它指定CREATE 或ALTER 語句是否執行檢查

INITIALLY Clause The INITIALLY clause establishes the default checking behavior for constraints that are DEFERRABLE. The INITIALLY setting can be overridden by a SET CONSTRAINT(S) statement in a subsequent transaction.

• Specify INITIALLY IMMEDIATE to indicate that Oracle should check this constraint at the end of each subsequent SQL statement. If you do not specify INITIALLY at all, then the default is INITIALLY IMMEDIATE.

  If you declare a new constraint INITIALLY IMMEDIATE, then it must be valid at the time the CREATE TABLE or ALTER TABLE statement is committed or the statement will fail.

• Specify INITIALLY DEFERRED to indicate that Oracle should check this constraint at the end of subsequent transactions.

This clause is not valid if you have declared the constraint to be NOT DEFERRABLE, because a NOT DEFERRABLE constraint is automatically INITIALLY IMMEDIATE and cannot ever be INITIALLYDEFERRED.