Sql

Tuple Variables

Disambiguating attributes by prefixing the relation name works as long as the query involves combining many different relations. On the other hand, sometimes we need to ask a query that involves two or more tuples from the same relation,

Subqueries

In SQL, one query can be used in many ways to help in the evaluation of another. A query that is part of another is called a subquery. Subqueries can have subqueries, and so on, down as many levels as we desire. We already saw one example of the use of subqueries; in

Conditions Involving Tuples

A tuple in SQL is represented by a parenthesized list of scalar values. Examples are (123, foo) and (name, address, networth). The first of these has constants as components; the second has attributes as components. Mixing of constants and attributes is permitted.

Defining a Relation Schema in SQL

Now we shall begin a discussion of data definition, the portions of SQL that involve explaining the structure of information in the database. On the contrary, the aspects of SQL discussed previously - queries and modifications - are often called data manipulation.

Simple Table Declarations

The simplest form of declaration of a relation schema comprises the keywords CREATE TABLE followed by the name of the relation and a parenthesized list of the attribute names and their types.

Default Values / Indexes

When we create or change tuples, we sometimes do not have values for all components. For instance, we mentioned in "Simple Table Declarations" Example 2 that when we add a column to a relation schema, the existing tuples do not have a known value, and it was

View Definitions

Relations that are defined with a CREATE TABLE statement in fact exist in the database. That is an SQL system stores tables in some physical organization. They are persistent, in the sense that they can be expected to exist indefinitely and not to change unless they are clearly told to

Modifying Views

In limited conditions it is possible to carry out an insertion, deletion, or update to a view. In the beginning, this idea makes no sense at all, since the view does not exist the way a base table (stored relation) does. What could it mean, say, to insert a new tuple into a view? Where

Queries in PSM

There are many ways that select-from-where queries are used in PSM. 1. Subqueries can be used in conditions, or in general, any place a subquery is legal in SQL. We saw two examples of subqueries in lines (3) and (6) of "Branching Statements" Figure 2, for instance.

Page 2 of 2 Previous 1 2 Next