【正文】 of 3NF allows certain functional dependencies that are not allowed in BCNF. A dependency α → β that satisfies only the third alternative of the 3NF definition is not allowed in BCNF, but is allowed in 3NF. Fourth Normal FormSome relation schemas, even though they are in BCNF, do not seem to be sufficiently normalized, in the sense that they still suffer from the problem of repetition of information. Consider again our banking example. Assume that, in an alternative design for the bank database schema, we have the schema BCschema = (loannumber, customername, customerstreet, customercity)The astute reader will recognize this schema as a nonBCNF schema because of the functional dependencycustomername → customerstreet customercitythat we asserted earlier, and because customername is not a key for BCschema. However, assume that our bank is attracting wealthy customers who have several addresses (say, a winter home and a summer home). Then, we no longer wish to enforce the functional dependency customername → customerstreet customercity. If we remove this functional dependency, we find BCschema to be in BCNF with respect to our modified set of functional dependencies. Yet, even though BCschema is now in BCNF, we still have the problem of repetition of information that we had earlier.To deal with this problem, we must define a new form of constraint, called a multivalued dependency. As we did for functional dependencies, we shall use multivalued dependencies to define a normal form for relation schemas. This normal form, called fourth normal form (4NF), is more restrictive than shall see that every 4NF schema is also in BCNF, but there are BCNF schemas that are not in 4NF. Definition of Fourth Normal Form蘇州大學(xué)本科生畢業(yè)設(shè)計（論文） XIIIConsider again our BCschema example in which the multivalued dependency customername→→customerstreet customercity holds, but no nontrivial functional dependencies hold. Although BCschema is in BCNF, the design is not ideal, since we must repeat a customer39。s where clause. At the other extreme, the where clause may be empty. The request delete from loan deletes all tuples from the loan relation. (Welldesigned systems will seek confirmation from the user before executing such a devastating request.)蘇州大學(xué)本科生畢業(yè)設(shè)計（論文） VIII InsertionTo insert data into a relation, we either specify a tuple to be inserted or write a query whose result is a set of tuples to be inserted. Obviously, the attribute values for inserted tuples must be members of the attribute39。 s fundamental constructs and concepts. Individual implementations of SQL may differ in details, or may support onl11a subset of the full language. BackgroundIBM developed the original version of SQL at its San Jose Research Laboratory (now the Almaden Research Center). IBM implemented the language, originally called Sequel, as part of the System R project in the early 1970s. The Sequel language has evolved since then, and its name has changed to SQL (Structured Query Language). Many products now support the SQL language. SQL has clearly established itself as the standard relationaldatabase language.In 1986, the American National Standards Institute (ANSI) and the International Organization for Standardization (ISO) published an SQL standard, called SQL86. IBM published its own corporate SQL standard, the Systems Application Architecture Database Interface (SAASQL) in 1987. ANSI published an extended standard for SQL, SQL89, in 1989. The next version of the standard was SQL92 standard, and the most recent version is SQL: 1999. 蘇州大學(xué)本科生畢業(yè)設(shè)計（論文） IIThe SQL language has several parts:? Datadefinition language (DDL). The SQL DDL provides mands for defining relation schemas, deleting relations, and modifying relation schemas.? Interactive datamanipulation language (DML). The SQL DML includes a query language based on both the relational algebra and the tuple relational calculus. It includes also mands to insert tuples into, delete tuples from, and modify tuples in the database.? View definition. The SQL DDL includes mands for defining views.? Transaction control. SQL includes mands for specifying the beginning and ending of transactions.? Embedded SQL and dynamic SQL. Embedded and dynamic SQL define how SQL statements can be embedded within generalpurpose programming languages, such as C, C++, Java, PL/I, Cobol, Pascal, and Fortran.? Integrity. The SQL DDL includes mands for specifying integrity constraints that the data stored in the database must satisfy. Updates that violate integrity constraints are disallowed.? Authorization. The SQL DDL includes mands for specifying access rights to relations and views. Basic StructureA relational database consists of a collection of relations, each of which is assigned a unique name. Each relation has a structure similar to that presented in Chapter 3. SQL allows the use of null values to indicate that the value either is unknown or does not exist. It allows a user to specify which attributes cannot be assigned null values. The basic structure of an SQL expression consists of three clauses: select, from, and where.? The select clause corresponds to the projection operation of the relational algebra. It is used to list the attributes desired in the result of a query.? The from clause corresponds to the Cartesianproduct operation of the relational algebra. It lists the relations to be scanned in the evaluation of the expression.? The where clause corresponds to the selection predicate of the relational algebra. It consists of a predicate involving attributes of the relations that 蘇州大學(xué)本科生畢業(yè)設(shè)計（論文） IIIappear in the from clause.That the term select has different meaning in SQL than in the relational algebra is an unfortunate historical fact. We emphasize the different interpretati