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As the relational database model started to become very popular in the mid-1980s, every DBMS vendor scrambled to describe their product as relational Some of these products had only a SQL-like query language layered on top of an underlying network or hierarchical database Some of them implemented only a very rudimentary table structure and no query language at all Soon the question of What is a true relational database became a topic of debate, and DBMS vendors began claiming that their products were more relational than the competition In 1985, Ted Codd, whose seminal technical article 15 years earlier had defined the relational data model, addressed this question in Computerworld, one of the leading trade publications.

This method creates a primary key on the ARTIST_ID and ARTIST_NAME columns, so that the combined values of both columns must be unique, although duplicates can exist within the individual column. An experienced database designer will quickly point out to you that this is a superkey, which means that it has more columns in it than the minimum needed to form a primary key. And that is true ARTIST_ID by itself is unique and we really don t need to add ARTIST_NAME to it in order to form a primary key, and we want to be sure that duplicate values of ARTIST_ID are not entered into the table, which means we should have a primary key with only ARTIST_ID in it. It was only done here to illustrate that a primary key can contain multiple columns and to define one that way, a table constraint must be used. You might find that you want to define both PRIMARY KEY and UNIQUE constraints in a table. To do so, you simply define the constraints as you normally would. For example, the following SQL statement defines a PRIMARY KEY constraint on the ARTIST_ID column and a UNIQUE constraint on the ARTIST_NAME column:

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In his two-part article, entitled Is Your DBMS Really Relational (October 14, 1985) and Does Your DBMS Run By the Rules (October 21, 1985), Codd presented 12 rules that a database must obey if it is to be considered truly relational: 1 Information rule All information in a relational database is represented explicitly at the logical level and in exactly one way by values in tables 2 Guaranteed access rule Each and every datum (atomic value) in a relational database is guaranteed to be logically accessible by resorting to a combination of table name, primary key value, and column name 3 Systematic treatment of NULL values.

NULL values (distinct from an empty character string or a string of blank characters and distinct from zero or any other number) are supported in a fully relational DBMS for representing missing information and inapplicable information in a systematic way, independent of the data type 4 Dynamic online catalog based on the relational model The database description is represented at the logical level in the same way as ordinary data, so that authorized users can apply the same relational language to its interrogation as they apply to the regular data 5 Comprehensive data sublanguage rule A relational system may support several languages and various modes of terminal use (for example, the fill-in-the-blanks mode).

CREATE TABLE CD_ARTISTS ( ARTIST_ID INT PRIMARY KEY, ARTIST_NAME VARCHAR(60), AGENCY_ID INT, CONSTRAINT UN_ARTIST_NAME UNIQUE (ARTIST_NAME) );

However, there must be at least one language whose statements are expressible, per some well-defined syntax, as character strings, and that is comprehensive in supporting all of the following items: Data definition View definition Data manipulation (interactive and by program) Integrity constraints Authorization Transaction boundaries (begin, commit, and rollback).

You would achieve the same results with the following code:

Part I:

As described throughout the earlier parts of this chapter, the SQL1 standard provides a subquery capability for expressing more complex database queries. The subquery takes the same form as a SQL query (that is, a SELECT statement), but a SQL1 subquery must be scalar-valued that is, it must produce a single data value as its query results. The value generated by the subquery is then used as part of an expression within the main SQL statement that contains the subquery. This use of subqueries is supported by the major enterprise-class relational database systems today. Subsequent versions of the SQL standard dramatically expand the subquery facility, including support for row-valued subqueries. A row-valued subquery returns not just a single data item, but a row of data items, which can be used in SQL expressions and compared

CREATE TABLE CD_ARTISTS ( ARTIST_ID INT, ARTIST_NAME VARCHAR(60) UNIQUE, AGENCY_ID INT, CONSTRAINT PK_ARTIST_ID PRIMARY KEY (ARTIST_ID) );

Part II:

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