Introduction to Structured Query Language

Introduction to Structured Query Language

EXISTS & ALL

EXISTS uses a subquery as a condition, where the condition is True if the subquery returns any rows, and False if the subquery does not return any rows; this is a nonintuitive feature with few unique uses. However, if a prospective customer wanted to see the list of Owners only if the shop dealt in Chairs, try:

SELECT OWNERFIRSTNAME, OWNERLASTNAME 
FROM ANTIQUEOWNERS 
WHERE EXISTS

(SELECT *  FROM ANTIQUES  WHERE ITEM = 'Chair');

If there are any Chairs in the Antiques column, the subquery would return a row or rows, making the EXISTS clause true, causing SQL to list the Antique Owners. If there had been no Chairs, no rows would have been returned by the outside query.

ALL is another unusual feature, as ALL queries can usually be done with different, and possibly simpler methods; let’s take a look at an example query:

SELECT BUYERID, ITEM 
FROM ANTIQUES  WHERE PRICE >= ALL
(SELECT PRICE FROM ANTIQUES);

This will return the largest priced item (or more than one item if there is a tie), and its buyer. The subquery returns a list of all Prices in the Antiques table, and the outer query goes through each row of the Antiques table, and if its Price is greater than or equal to every (or ALL) Prices in the list, it is listed, giving the highest priced Item. The reason "=" must be used is that the highest priced item will be equal to the highest price on the list, because this Item is in the Price list.

UNION & Outer Joins (briefly explained)

There are occasions where you might want to see the results of multiple queries together, combining their output; use UNION. To merge the output of the following two queries, displaying the ID’s of all Buyers, plus all those who have an Order placed:

SELECT BUYERID 
FROM ANTIQUES 
UNION 
SELECT OWNERID 
FROM ORDERS;

Notice that SQL requires that the Select list (of columns) must match, column-by-column, in data type. In this case BuyerID and OwnerID are of the same data type (integer). Also notice that SQL does automatic duplicate elimination when using UNION (as if they were two "sets"); in single queries, you have to use DISTINCT.

The outer join is used when a join query is "united" with the rows not included in the join, and are especially useful if constant text "flags" are included. First, look at the query:

SELECT OWNERID, 'is in both Orders & Antiques' 
FROM ORDERS, ANTIQUES 
WHERE OWNERID = BUYERID 
UNION 
SELECT BUYERID, 'is in Antiques only' 
FROM ANTIQUES 
WHERE BUYERID NOT IN

(SELECT OWNERID FROM ORDERS);

The first query does a join to list any owners who are in both tables, and putting a tag line after the ID repeating the quote. The UNION merges this list with the next list. The second list is generated by first listing those ID’s not in the Orders table, thus generating a list of ID’s excluded from the join query. Then, each row in the Antiques table is scanned, and if the BuyerID is not in this exclusion list, it is listed with its quoted tag. There might be an easier way to make this list, but it’s difficult to generate the informational quoted strings of text.

This concept is useful in situations where a primary key is related to a foreign key, but the foreign key value for some primary keys is NULL. For example, in one table, the primary key is a salesperson, and in another table is customers, with their salesperson listed in the same row. However, if a salesperson has no customers, that person’s name won’t appear in the customer table. The outer join is used if the listing of all salespersons is to be printed, listed with their customers, whether the salesperson has a customer or not–that is, no customer is printed (a logical NULL value) if the salesperson has no customers, but is in the salespersons table. Otherwise, the salesperson will be listed with each customer.

Another important related point about Nulls having to do with joins: the order of tables listed in the From clause is very important. The rule states that SQL "adds" the second table to the first; the first table listed has any rows where there is a null on the join column displayed; if the second table has a row with a null on the join column, that row from the table listed second does not get joined, and thus included with the first table’s row data. This is another occasion (should you wish that data included in the result) where an outer join is commonly used. The concept of nulls is important, and it may be worth your time to investigate them further.

Walid BaniHani

 

College of Applied Sciences - Al Rustaq

Leave a Reply

Your email address will not be published. Required fields are marked *