合并拉取请求时的 Jira Automation 规则
Posted by: AJ Welch
Much to the frustration of database administrators worldwide, prior to Oracle version 12c in mid-2014, Oracle simply had no inherent ability to inherently generate auto incrementing columns within a table schema. While the reasons for this design decision can only be guessed at, the good news is that even for users on older Oracle systems, there is a possible workaround to circumnavigate this pitfall and create your own auto incremented primary key column.
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The first step is to create a SEQUENCE
in your database, which is a data object that multiple users can access to automatically generate incremented values. As discussed in the documentation, a sequence in Oracle prevents duplicate values from being created simultaneously because multiple users are effectively forced to “take turns” before each sequential item is generated.
For the purposes of creating a unique primary key for a new table, first we must CREATE
the table we’ll be using:
CREATE TABLE books (
id NUMBER(10) NOT NULL,
title VARCHAR2(100) NOT NULL
);
Next we need to add a PRIMARY KEY
constraint:
ALTER TABLE books
ADD (
CONSTRAINT books_pk PRIMARY KEY (id)
);
Finally, we’ll create our SEQUENCE
that will be utilized later to actually generate the unique, auto incremented value.
CREATE SEQUENCE books_sequence;
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While we have our table created and ready to go, our sequence is thus far just sitting there but never being put to use. This is where TRIGGERS
come in.
Similar to an event
in modern programming languages, a TRIGGER
in Oracle is a stored procedure that is executed when a particular event occurs.
Typically a TRIGGER
will be configured to fire when a table is updated or a record is deleted, providing a bit of cleanup when necessary.
In our case, we want to execute our TRIGGER
prior to INSERT
into our books
table, ensuring our SEQUENCE
is incremented and that new value is passed onto our primary key column.
CREATE OR REPLACE TRIGGER books_on_insert
BEFORE INSERT ON books
FOR EACH ROW
BEGIN
SELECT books_sequence.nextval
INTO :new.id
FROM dual;
END;
Here we are creating (or replacing if it exists) the TRIGGER
named books_on_insert
and specifying that we want the trigger to fire BEFORE INSERT
occurs for the books
table, and to be applicable to any and all rows therein.
The ‘code’ of the trigger itself is fairly simple: We SELECT
the next incremental value from our previously created books_sequence SEQUENCE
, and inserting that into the :new
record of the books
table in the specified .id
field.
Note: The FROM dual
part is necessary to complete a proper query but is effectively irrelevant. The dual
table is just a single dummy row of data and is added, in this case, just so it can be ignored and we can instead execute the system function of our trigger rather than returning data of some kind.
IDENTITY columns
IDENTITY
columns were introduced in Oracle 12c, allowing for simple auto increment functionality in modern versions of Oracle.
Using the IDENTITY
column is functionally similar to that of other database systems. Recreating our above books
table schema in modern Oracle 12c or higher, we’d simply use the following column definition.
CREATE TABLE books (
id NUMBER GENERATED BY DEFAULT ON NULL AS IDENTITY,
title VARCHAR2(100) NOT NULL
);