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The same was true for UNIQUE constraints. It wasn t until SQL-92 that null values were permitted in columns included in a UNIQUE constraint, which clearly set them apart from PRIMARY KEY constraints. Also, be wary of variations across vendor implementations. For example, Oracle will automatically add NOT NULL constraints to columns included in a PRIMARY KEY constraint, while SQL Server (or at least some versions of it) will display an error if you attempt to create a PRIMARY KEY constraint using columns that have not been specified with a NOT NULL constraint.

The most common use of row-valued expressions in the WHERE or HAVING clause is within a test for equality, as illustrated by the last few examples. A constructed row (often consisting of column values from a candidate row of query results) is compared with another constructed row (perhaps a row of subquery results or a row of literal values), and if the rows are equal, the candidate row is included in the query results. The SQL standard also provides for row-valued forms of the inequality comparison tests and the range test. When comparing two rows for inequality, SQL uses the same rules that it would use if the columns were being used to sort the rows. It compares the contents of the first column in the two rows, and if they are unequal, uses them to order the rows. If they are equal, the comparison moves to the second column, and then the third, and so on. Here are the resulting comparisons for some three-column constructed rows derived from the ORDERS table: ('ACI','41002',54) < ('REI','2A44R',5) based on rst column ('ACI','41002',54) < ('ACI','41003',35) based on second column ('ACI','41002',10) < ('ACI','41002',54) based on third column

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11:

Up to this point, the types of constraints that I ve discussed have had to do primarily with ensuring the integrity of data within a table. The NOT NULL constraint prevents the use of null values within a column, and the UNIQUE and PRIMARY KEY constraints ensure the uniqueness of values within a column or set of columns. However, the FOREIGN KEY constraint is different in that it is concerned with how data in one table relates to data in another table, which is why it is known as a referential constraint it references another table. (Actually, there is an exception called a recursive relationship where the foreign key refers to another row in the same table, but I m going to ignore this special case for now in order to focus on the basics.) You might recall from 1 that tables in a relational database are linked together in a meaningful way in order to ensure the integrity of the data. This association between tables forms a relationship that provides referential integrity between tables. Referential integrity prevents the manipulation of data in one table from adversely affecting data in another table. Let s take a look at an example that illustrates this point. Figure 4-5 shows two tables (CD_TITLES and CD_PUBLISHERS) that are each defined with a primary key.

4 discussed primary keys, foreign keys, and the parent/child relationships that they create between tables. Figure 11-1 shows the SALESREPS and OFFICES tables and illustrates once again how foreign keys and primary keys work. The OFFICE column is the primary key for the OFFICES table, and it uniquely identifies each row. The REP_OFFICE column in the SALESREPS table is a foreign key for the OFFICES table. It identifies the office where each salesperson is assigned. The REP_OFFICE and OFFICE columns create a parent/child relationship between the OFFICES and SALESREPS rows. Each OFFICES (parent) row has zero or more SALESREPS (child) rows with matching office numbers. Similarly, each SALESREPS (child) row has exactly one OFFICES (parent) row with a matching office number. Suppose you tried to insert a new row into the SALESREPS table that contained an invalid office number, as in this example:

CD_TITLES CD_TITLE_ID: CD_TITLE: INT VARCHAR(60) 11001 11002 11003 11004 11005 11006 Famous Blue Raincoat Blue Past Light Kojiki That Christmas Feeling PUBLISHER_ID: INT 5422 5402 5412 5409 5403 CD_PUBLISHERS PUBLISHER_ID: COMPANY_NAME: INT VARCHAR(60) 5403 5402 5409 5412 5422 MCA Records Reprise Records Geffen Windham Hill Records Private Music

INSERT INTO SALESREPS (EMPL_NUM, NAME, REP_OFFICE, AGE, HIRE_DATE, SALES) VALUES (115, 'George Smith', 31, 37, '2008-04-01', 0.00);

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