Introduction to Structured Query Language

Introduction to Structured Query Language

IN & BETWEEN

An easier method of using compound conditions uses IN or BETWEEN. For example, if you wanted to list all managers and staff:

SELECT EMPLOYEEIDNO 
FROM EMPLOYEESTATISTICSTABLE 
WHERE POSITION IN ('Manager', 'Staff');

or to list those making greater than or equal to $30,000, but less than or equal to $50,000, use:

SELECT EMPLOYEEIDNO 
FROM EMPLOYEESTATISTICSTABLE 
WHERE SALARY BETWEEN 30000 AND 50000;

To list everyone not in this range, try:

SELECT EMPLOYEEIDNO 
FROM EMPLOYEESTATISTICSTABLE 
WHERE SALARY NOT BETWEEN 30000 AND 50000;

Similarly, NOT IN lists all rows excluded from the IN list.

Additionally, NOT’s can be thrown in with AND’s & OR’s, except that NOT is a unary operator (evaluates one condition, reversing its value, whereas, AND’s & OR’s evaluate two conditions), and that all NOT’s are performed before any AND’s or OR’s.

SQL Order of Logical Operations (each operates from left to right)

  1. NOT
  2. AND
  3. OR

Using LIKE

Look at the EmployeeStatisticsTable, and say you wanted to see all people whose last names started with “S”; try:

 SELECT EMPLOYEEIDNO 
FROM EMPLOYEEADDRESSTABLE 
WHERE LASTNAME LIKE 'S%';

The percent sign (%) is used to represent any possible character (number, letter, or punctuation) or set of characters that might appear after the “S”. To find those people with LastName’s ending in “S”, use ‘%S’, or if you wanted the “S” in the middle of the word, try ‘%S%’. The ‘%’ can be used for any characters in the same position relative to the given characters. NOT LIKE displays rows not fitting the given description. Other possibilities of using LIKE, or any of these discussed conditionals, are available, though it depends on what DBMS you are using; as usual, consult a manual or your system manager or administrator for the available features on your system, or just to make sure that what you are trying to do is available and allowed. This disclaimer holds for the features of SQL that will be discussed below. This section is just to give you an idea of the possibilities of queries that can be written in SQL.

Joins

In this section, we will only discuss inner joins, and equijoins, as in general, they are the most useful. For more information, try the SQL links at the bottom of the page.

Good database design suggests that each table lists data only about a single entity, and detailed information can be obtained in a relational database, by using additional tables, and by using a join.

First, take a look at these example tables:

AntiqueOwners

OwnerIDOwnerLastNameOwnerFirstName
01JonesBill
02SmithBob
15LawsonPatricia
21AkinsJane
50FowlerSam

Orders

OwnerIDItemDesired
02Table
02Desk
21Chair
15Mirror

Antiques

SellerIDBuyerIDItem
0150Bed
0215Table
1502Chair
2150Mirror
5001Desk
0121Cabinet
0221Coffee Table
1550Chair
0115Jewelry Box
0221Pottery
2102Bookcase
5001Plant Stand

Performing a Join

The purpose of these keys is so that data can be related across tables, without having to repeat data in every table–this is the power of relational databases. For example, you can find the names of those who bought a chair without having to list the full name of the buyer in the Antiques table…you can get the name by relating those who bought a chair with the names in the AntiqueOwners table through the use of the OwnerID, which relates the data in the two tables. To find the names of those who bought a chair, use the following query:

SELECT OWNERLASTNAME, OWNERFIRSTNAME
FROM ANTIQUEOWNERS, ANTIQUES
WHERE BUYERID = OWNERID AND ITEM = 'Chair';

Note the following about this query…notice that both tables involved in the relation are listed in the FROM clause of the statement. In the WHERE clause, first notice that the ITEM = ‘Chair’ part restricts the listing to those who have bought (and in this example, thereby own) a chair. Secondly, notice how the ID columns are related from one table to the next by use of the BUYERID = OWNERID clause. Only where ID’s match across tables and the item purchased is a chair (because of the AND), will the names from the AntiqueOwners table be listed. Because the joining condition used an equal sign, this join is called an equijoin. The result of this query is two names: Smith, Bob & Fowler, Sam.

Dot notation refers to prefixing the table names to column names, to avoid ambiguity, as follows:

SELECT ANTIQUEOWNERS.OWNERLASTNAME, ANTIQUEOWNERS.OWNERFIRSTNAME
FROM ANTIQUEOWNERS, ANTIQUES
WHERE ANTIQUES.BUYERID = ANTIQUEOWNERS.OWNERID AND ANTIQUES.ITEM = 'Chair';

As the column names are different in each table, however, this wasn’t necessary.

Walid BaniHani

 

College of Applied Sciences - Al Rustaq

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