are invisible links between two databases that
situation that exists between two relational databases tables when
one table has a foreign key that references the primary key of the
other table. Relationships allow databases to split and store data in
different tables while still being connected.
of the relational model of
Advantages of a Relational Database is a database structured on the
model. Data are stored and presented in a tabular format, organized
in rows and columns with one record per row. This
helps a lot having the relational model as there is no duplicated
data and helps a lot with organising tables.
Normalisation is the
process of organizing data in a way that the results make the data
easier for you find individual tables. This also stops data
duplication because you would create a table in between the 2 tables
that have the same data.
data type when
data into groups of similar things. For example
is a data type that is used to store
text and an integer is a data type used to classify whole numbers.
used to describe the amount of data that can go in 1 field.
All data entry fields have a default maximum size
is normally set to 12.
For example a single line input field often has a 255 character limit
other things may vary.
rule is a criteria
or constraint used in the database
on a field that limits what the user can type into that field. One a
validation rule is set anything that is not included in it will be
not allowed to enter into
primary key is a special thing
attached to a field that
to uniquely identify all table records. A primary key’s main features
are: It must contain a unique value for each row of data. It
is also what you link when linking 2 tables together.
foreign key is a field (or sometimes
of fields) in one table that uniquely identifies a row of another
table or the same table but
a different position.
foreign key is defined in a second table but it refers to the primary
key in the first table.
integrity is a relational only
which states that relationships between
must always be consistent. In other words, any foreign key field must
agree with the primary key that is referenced by the foreign key.
Auto incremental keys
allows a unique number to be generated automatically when a new
record is inserted into a table. Often this is the primary key field
that we would like to be created automatically every time a new
record is inserted. And
example of this is when you are creating character each new character
has there own unique ID number.
a characteristic linked
to a field.
attribute refers to a database component, such as a table. It also
may refer to a database field.
redundancy is a
created within a database in which the same piece of data is held in
two separate places. Whenever
data is repeated, this is
data redundancy. This happens
sometimes happen when your writing lots of data and can occur
by accident but is also done deliberately when
your testing different fields and stuff to check that they work.