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Tuples in tuple sets handle the same role as rows in tables for relational databases. Tuples have an arbitrary number of fields, and fields do not need names. Every tuple in a database has one (unique not null) primary key and one or more secondary keys, which are enabled for immediate lookup via indexes.
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In SQL, the unique keys have a UNIQUE constraint assigned to them in order to prevent duplicates (a duplicate entry is not valid in a unique column). Alternate keys may be used like the primary key when doing a single-table select or when filtering in a where clause, but are not typically used to join multiple tables.
A table is a sequence, optionally supplemented by named keys: digit["two"]="2". Several table functions like table.concat will only work with the numbered values and ignore named keys. The metatable offers a large, optional set of methods for altering table behavior. For example, you can define a table to be callable like a function.
A table (called the referencing table) can refer to a column (or a group of columns) in another table (the referenced table) by using a foreign key. The referenced column(s) in the referenced table must be under a unique constraint, such as a primary key. Also, self-references are possible (not fully implemented in MS SQL Server though [5]).
Surrogate keys simplify the creation of foreign key relationships because they only require a single column (as opposed to composite keys - which require multiple columns). When creating a query on the database, forgetting to include all the columns in a composite foreign key when joining tables can lead to unexpected results in the form of an ...
In the relational model of databases, a primary key is a designated attribute that can reliably identify and distinguish between each individual record in a table.The database creator can choose an existing unique attribute or combination of attributes from the table (a natural key) to act as its primary key, or create a new attribute containing a unique ID that exists solely for this purpose ...
Takes an array t with one or more nil values, and removes the nil values while preserving the order, so that the array can be safely traversed with ipairs. Any keys that are not positive integers are removed. For example, for the table {1, nil, foo = 'bar', 3, 2}, compressSparseArray will return {1, 3, 2}.