MariaDB 10.3 support Oracle mode sequences

Sequences are used to requesting unique values on demand, The best use case of sequences is to have a unique ID. , that can be used across multiple tables. In some cases sequences are really helpful to have an identifier before an actual row is inserted. With the normal way of having an automatically incrementing identifier, the identifier value will only be available after insert of the row and the identifier will only be unique inside its own table. MariaDB Server 10.3 follows the standard and includes compatibility with the way Oracle does sequences introduced in Oracle Database Server on top of the standard.

Simple steps to create a sequence in MariaDB 10.3 onwards, a create statement is used:

MariaDB [MDB101]> CREATE SEQUENCE Seq1_100
    -> START WITH 100
    -> INCREMENT BY 1;
Query OK, 0 rows affected (0.015 sec)

This creates a sequence that starts at 100 and is incremented with 1 every time a value is requested from the sequence. The sequence will be visible among the tables in the database, i.e. if you run SHOW TABLES it will be there. You can use DESCRIBE on the sequence to see what columns it has.

To test out the usage of sequences let’s create a table:

MariaDB [MDB101]> CREATE TABLE TAB1 (
    -> Col1 int(10) NOT NULL,
    -> Col2 varchar(30) NOT NULL,
    -> Col3 int(10) NOT NULL,
    ->  PRIMARY KEY (Col1)
    -> );
Query OK, 0 rows affected (0.018 sec)

Since we want to use sequences this time, we did not put AUTO_INCREMENT on the Col1 column. Instead we will ask for the next value from the sequence in the INSERT statements:

MariaDB [MDB101]> INSERT INTO TAB1 (Col1, Col2, Col3) VALUES (NEXT VALUE FOR Seq1_100, 'India', 10);
Query OK, 1 row affected (0.011 sec)

MariaDB [MDB101]> INSERT INTO TAB1 (Col1, Col2, Col3) VALUES (NEXT VALUE FOR Seq1_100, 'Jakarta', 20);
Query OK, 1 row affected (0.008 sec)

MariaDB [MDB101]> INSERT INTO TAB1 (Col1, Col2, Col3) VALUES (NEXT VALUE FOR Seq1_100, 'Singapore', 20); 
Query OK, 1 row affected (0.016 sec)

MariaDB [MDB101]> INSERT INTO TAB1 (Col1, Col2, Col3) VALUES (NEXT VALUE FOR Seq1_100, 'Japan', 30);
Query OK, 1 row affected (0.007 sec)

Instead of having the NEXT VALUE FOR in each INSERT statement, it could have been the default value of the column in this way:

MariaDB [MDB101]> ALTER TABLE TAB1 MODIFY Col1 int(10) NOT NULL DEFAULT NEXT VALUE FOR Seq1_100;
Query OK, 0 rows affected (0.007 sec)
Records: 0  Duplicates: 0  Warnings: 0

Running a SELECT over the TAB1 table will look like this:

MariaDB [MDB101]> SELECT * FROM TAB1;;
+------+-----------+------+
| Col1 | Col2      | Col3 |
+------+-----------+------+
|  100 | India     |   10 |
|  101 | Jakarta   |   20 |
|  102 | Singapore |   20 |
|  103 | Japan     |   30 |
+------+-----------+------+
4 rows in set (0.000 sec)

As we can see the Col1 column has been populated with numbers that start from 100 and are incremented with 1 as defined in the sequence’s CREATE statement. To get the last retrieved number from the sequence PREVIOUS VALUE is used:

MariaDB [MDB101]> SELECT PREVIOUS VALUE FOR Seq1_100;
+-----------------------------+
| PREVIOUS VALUE FOR Seq1_100 |
+-----------------------------+
|                         103 |
+-----------------------------+
1 row in set (0.000 sec)

MariaDB 10.3 shipped another very useful option for sequences is CYCLE, which means that we start again from the beginning after reaching a certain value. For example, if there are 5 phases in a process that are done sequentially and then start again from the beginning, we could easily create a sequence to always be able to retrieve the number of the next phase:

MariaDB [MDB101]> CREATE SEQUENCE Seq1_100_c5 
    ->   START WITH 100
    ->   INCREMENT BY 1
    -> MAXVALUE = 200
    -> CYCLE;
Query OK, 0 rows affected (0.012 sec)

The sequence above starts at 100 and is incremented with 1 every time the next value is requested. But when it reaches 200 (MAXVALUE) it will restart from 100 (CYCLE).

We can also set the next value of a sequence, to ALTER a sequence or using sequences in Oracle mode with Oracle specific syntax. To switch to Oracle mode use:

MariaDB [MDB101]> SET SQL_MODE=ORACLE;
Query OK, 0 rows affected (0.000 sec)

After that you can retrieve the next value of a sequence in Oracle style:

MariaDB [MDB101]> SELECT Seq1_100.nextval;
+------------------+
| Seq1_100.nextval |
+------------------+
|              104 |
+------------------+
1 row in set (0.009 sec)

You can read about MariaDB sequences in the documentation, MariaDB documentation

 

The post MariaDB 10.3 support Oracle mode sequences appeared first on MySQL Consulting, Support and Remote DBA Services By MinervaDB.

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