EnterpriseDB: TABLE Oracle Tips by Burleson Consulting

As we have discussed several times, a table is a structure that holds rows (or records) of data.  A table is composed of columns, each column has a data type and each column can hold some amount of data of that data type.

Tables in EnterpriseDB are stored as files in the file system.  A single table is a single file.

TABLESPACES and SCHEMAs

I spoke of tablespaces and schemas in Chapter 2, The EnterpriseDB Platform.  A tablespace is a pointer to a directory and a schema is a grouping of database objects.  A tablespace or schema in EnterpriseDB is conceptually similar to the same in Oracle but is implemented quite a bit differently.  I will show an example using tablespaces and schemas below but if you don't quite understand that use, don't worry about it at this point.  If you ever do need the functionality they provide, the use will become obvious.

DEFAULT Expression

A Default column expression allows you to force a value in a table if the value is not provided in an insert statement. 

NULL|NOT NULL

You may set a column as NULL or NOT NULL.  A NOT NULL column will not allow the column to have no value.  Many times, a DEFAULT is used with a NOT NULL to ensure a value.  A NULL means that the column can be empty.  NULL is the default.

UNIQUE

The UNIQUE keyword identifies a unique constraint.  A unique constraint forces the column (or a combination of columns) to be unique for all records in the table.

A Unique constraint may be declared at the table level or at the column level.  At the column level, the unique constraint may reference only that single column.  At the table level, a unique constraint may reference multiple columns.

PRIMARY KEY

A Primary Key constraint is much like a unique constraint.  The differences are that a PRIMARY KEY (a single column or combination of columns) must be NOT NULL and Unique.  You may define only a single Primary Key for any particular table.

A Primary Key constraint creates a Unique Index on the column(s) you specify.

CHECK

A Check constraint allows you to validate data without writing code.  If you want to ensure that a column can only contain a certain selection of data, a check constraint is the best way to do it.  A check constraint must evaluate to a boolean value, i.e. 1 = 1 or 1 = 2.  The first would evaluate to TRUE (1 equals 1) and the second would evaluate to false (1 equals 2).  1 + 1 would not be a valid check constraint because it does not evaluate to TRUE or FALSE.

CONSTRAINT

The constraint keyword allows you to name a constraint.  You may use CONSTRAINT with a Unique, Primary Key, NULL|NOT NULL, or Check constraint to name it.  If you do not provide a name, the database will generate a name for you.

REFERENCES

A reference is a foreign key constraint.  You would use a reference to relate a child table to its parent table.  In our example earlier, the EMP table is a child of DEPT.  There is a reference between EMP.deptno and DEPT.deptno.  A table level foreign key reference on EMP would look like: FOREIGN KEY (deptno) REFERENCES dept (deptno).

This is an excerpt from the book "EnterpriseDB: The Definitive Reference" by Rampant TechPress.