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## Constraint naming standard

By Michael Bryzek

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### The Big Picture

Constraint naming standard is important for one reason: The SYS_* name oracle assigns to unnamed constraints is not very understandable. By correctly naming all contraints, we can quickly associate a particular constraint with our data model. This gives us two real advantages:

- We can quickly identify and fix any errors.

- We can reliabily modify or drop constraints

Why do we need a naming convention?

Oracle limits names, in general, to 30 characters, which is hardly enough for a human readable constraint name.

### Abbreviations

We propose the following naming convention for all constraints, with the following abbreviations taken from Oracle Docs at http://oradoc.photo.net/ora81/DOC/server.815/a67779/ch4e.htm#8953. Note that we shortened all of the constraint abbrevations to two characters to save room.

Constraint type | Abbreviation
--- | ---
references (foreign key) | fk
unique | un
primary key | pk
check | ck
not null | nn

### Format of constraint name

<table name>_<column_name>_<constraint abbreviation>

In reality, this won't be possible because of the character limitation on names inside oracle. When the name is too long, we will follow these two steps in order:

1. Abbreviate the table name with the table's initials (e.g. users -> u and users_contact -> uc).

2. Truncate the column name until it fits.

If the constraint name is still too long, you should consider rewriting your entire data model :)

Notes:

- If you have to abbreviate the table name for one of the constraints, abbreviate it for all the constraints

- If you are defining a multi column constraint, try to truncate the two column names evenly

### Example

    create table example_topics (
           topic_id    integer
    		   constraint example_topics_topic_id_pk
    		   primary key
    );

    create table constraint_naming_example (
           example_id		      integer
    				      constraint cne_example_id_pk
    				      primary key,
           one_line_description	      varchar(100)
    				      constraint cne_one_line_desc_nn
    				      not null,
           body			      clob,
           up_to_date_p		      char(1) default('t')
    				      constraint cne_up_to_date_p_check
    				      check(up_to_date_p in ('t','f')),
           topic_id			      constraint cne_topic_id_nn not null
    				      constraint cne_topic_id_fk references example_topics,
           -- Define table level constraint
           constraint cne_example_id_one_line_unq unique(example_id, one_line_description)
    );

### Why it's good to name primary keys

Naming primary keys might not have any obvious advantages. However, here's an example where naming the primary key really helps (and this is by no means a rare case!

    SQL> set autotrace traceonly explain;

    SQL> select * from constraint_naming_example, example_topics
    where constraint_naming_example.topic_id = example_topics.topic_id;

    Execution Plan
    ----------------------------------------------------------
       0	  SELECT STATEMENT Optimizer=CHOOSE
       1	0   NESTED LOOPS
       2	1     TABLE ACCESS (FULL) OF 'CONSTRAINT_NAMING_EXAMPLE'
       3	1     INDEX (UNIQUE SCAN) OF 'EXAMPLE_TOPICS_TOPIC_ID_PK' (UNI
    	  QUE)

Isn't it nice to see "EXAMPLE_TOPICS_TOPIC_ID_PK" in the trace and know exactly which table oracle is using at each step?

### Naming not null constraints is optional...

People disagree on whether or not we should be naming not null constraints. So, if you want to name them, please do so and follow the above naming standard. But, naming not null constraints is not a requirement.

About Naming the not null constraints

Though naming "not null" constraints doesn't help immeditately in error debugging (e.g. the error will say something like "Cannot insert null value into column"), we recommend naming not null constraints to be consistent in our naming of all constraints.

($Id: eng-standards-constraint-naming.html,v 1.42.2.3 2009/09/11 23:41:26 donb Exp $)

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