CS2 Current Technologies Note 2 CS2Bh

Transcription

1 CS2 Current Technologies Note 2 SQL Programming Basics Introduction We previously introduced the SQL programming language for relational database systems. We will now begin to examine some of SQL s more advanced features. We will primarily restrict ourselves to that part of SQL known as the data manipulation language (DML). The DML addresses the extraction of data from existing tables and their editing (i.e., the updating of table attribute values and adding/deleting of table rows/columns. SQL s data definition language (DDL) supports the definition of the logical and physical structure of databases. The DDL will be explained in detail in CS3. Testing for Multiple Conditions The WHERE clause described above allows us to test for one condition only. This is often not enough. Let s say we want to know the names of all clerks in department 20. To find out who they are, we have to use a WHERE clause that tests for two conditions: whether DEPTNO is equal 20, and whether JOB is equal to CLERK. SQL allows us to specify multiple conditions in the WHERE clause, using the AND connector. The general format of a WHERE clause with multiple conditions is as follows. WHERE condition 1 AND condition 2 AND AND condition n We illustrate this in Example 1, Example 1 List names of all CLERKs who work in department 20 SELECT ENAME FROM EMP WHERE DEPTNO = 20 AND JOB = CLERK ; ENAME SMITH ADAMS 1

2 Testing for Alternative Conditions When using the AND connector, a row is retrieved if it satisfies all conditions connected by AND. Many times, however, we are interested in rows if they satisfy at least one of a number of conditions, but not necessarily all. For example, we would like to know the names of all analysts and managers in the company. To retrieve information of this kind, SQL provides us with the OR connector in the WHERE clause. Its general form looks as follows. WHERE condition 1 OR condition 2 OR OR condition n Example 2 illustrates the use of this construct. Example 2 List the names of all managers and analysts. SELECT ENAME FROM EMP WHERE JOB = MANAGER OR JOB = ANALYST ; ENAME JONES BLAKE CLARK SCOTT FORD Testing for Values in a List Special cases of alternative search conditions are WHERE clauses which test for several values of one column, e.g. the query in Figure 2. Instead of writing a number of equality tests connected by OR, we can use the IN operator. The general form of a WHERE clause using IN is as follows. WHERE column-name IN ( list of values ) We can now rewrite the query in Figure 2 using the IN operator. query is given in Example 3. The new Example 3 List the names of all managers and analysts. SELECT ENAME FROM EMP WHERE JOB IN ( MANAGER, ANALYST ); (The output is the same as the table in Example 2.) 2

3 Testing for Negative Conditions Yet another form of the WHERE clause allows us to retrieve all those rows of a table which do NOT satisfy search conditions in the clause. To do so, we simply use!= instead of =. This construct is illustrated in Example 4. If executed, this query tells us that no records were found, which is to be expected since all analysts work in Department 20. Example 4 List names of analysts who are not working in department 20. SELECT ENAME FROM EMP WHERE JOB = ANALYST AND DEPTNO!= 20; NOTE: Instead of!= we can also use <> or ˆ=. Testing For A Range In all previous examples we used the WHERE clause to test for particular column values. SQL allows us, alternatively, to use search conditions which specify certain ranges of values. The four comparison operators used for range queries are: > for greater than, >= for greater than or equal to, < for less than, and <= for less than or equal to. The query in Example 5 illustrates the use of a comparison operator. Example 5 List employees with salaries higher than SELECT ENAME FROM EMP WHERE SAL > 2500; ENAME JONES BLAKE SCOTT KING FORD If a range query specifies an upper and a lower value for a column, we can use the BETWEEN operator instead of two conditions with comparison operators. The general form of the WHERE clause using the BETWEEN operator is as follows. WHERE column-name BETWEEN lower value AND upper value Example 6 illustrates the use of this construct. Looking at the output from this query, we find that only salespeople and clerks have salaries that fall in this range. We also note that there are many duplicates in the output. In general we are not interested in such duplicates and SQL allows us to suppress them using the DISTINCT keyword in the SELECT clause. 3

4 Example 6 Which jobs are paid less than 2000 but more than 1000? SELECT JOB FROM EMP WHERE SAL BETWEEN 1000 AND 2000; JOB SALESMAN SALESMAN SALESMAN SALESMAN CLERK CLERK Arithmetic Expressions Besides selecting certain columns and rows, SQL provides a number of functions which allow us to create new information based on existing tables. Given table EMP we know the salary and the commission of each salesman, but the total income is not explicitly stated. Obviously, it is easy to calculate it by adding up salary and commission for each salesman. The query in Example 7 shows how to add two column values using SQL. Example 7 What is the total remuneration of the salespeople? SELECT ENAME, SAL+COMM FROM EMP WHERE JOB = SALESMAN ; ENAME SAL+COMM ALLEN 1900 WARD 1750 MARTIN 2650 TURNER 1500 Although SAL+COMM is not a column of table EMP, we can use it in a SE- LECT statement as if it were originally in the table, e.g. we can use it to specify WHERE clause conditions or base the output order on it. A list of the most commonly used arithmetic operators is given below. Operator Meaning Operator Meaning + Addition POWER Exponentiation Subtraction ROUND Rounding Multiplication TRUNC Truncation / Division ABS Absolute Value 4

5 Ordering Rows of Query Result Table We have seen earlier how we can influence the order of columns in the output of a query. Now, we will see how the order of rows can be changed by using the ORDER BY clause. If this clause is not specified in the SELECT statement, SQL decides internally upon the order of rows presented in a query output. Using ORDER BY, however, the rows of the output can be ordered according to column values. Example 8 illustrates the use of the ORDER BY clause. Example 8 List all employees in department 10, ordered by their salary SELECT * FROM EMP WHERE DEPTNO = 10 ORDER BY SAL; EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO 7934 MILLER CLERK JAN CLARK MANAGER JUN KING PRESIDENT 17-NOV If more than one column name is given in the ORDER BY clause, SQL sorts on the first given column, then on the second given column, and so on. By default rows are sorted in ascending order. This can be overridden by placing the keyword DESC after the column in the ORDER BY clause which we would like to be listed in descending order. Example 9 List all employees in the order of their jobs, and within that order in descending salary order. SELECT JOB, SAL, ENAME FROM EMP ORDER BY JOB, SAL DESC; JOB SAL ENAME ANALYST 3000 SCOTT ANALYST 3000 FORD CLERK 1300 MILLER CLERK 1100 ADAMS CLERK 950 JAMES CLERK 800 SMITH MANAGER 2975 JONES MANAGER 2850 BLAKE MANAGER 2450 CLARK PRESIDENT 5000 KING SALESMAN 1600 ALLEN SALESMAN 1500 TURNER SALESMAN 1250 WARD SALESMAN 1250 MARTIN 5

6 Functions on Groups of Rows Although the SELECT statement, as presented so far, has given us quite a powerful tool for querying a database, there are still a number of questions we may want to ask, but which we would find hard to formulate with our current knowledge of the SELECT statement. For example, we may be interested to know what is the highest salary in the company and who earns it, what is the lowest salary, what is the average salary, what is the total amount of commission paid to salesmen, how many managers are employed, and so on. The common factor of all these questions is that they can only be answered by looking at all rows of a table (EMP) as a single entity, rather than treating rows as individual pieces of information. The following functions are available. AVG( column-name ): Average (arithmetic mean) value of a column; COUNT( column-name ): Number of rows with a value not null for that column; MAX( column-name ): Maximum value of a column; MIN( column-name ): Minimum value of a column; SUM( column-name ): Sum of values of a column; Contrast these with the arithmetic functions described in Note 3. Those function were applied to individual values, whereas these are applied to sets of values. The following examples will demonstrate the use of these functions. Example 10? What is the average salary of all clerks employed in this company SELECT AVG(SAL) FROM EMP WHERE JOB = CLERK ; AVG(SAL) Example 11 How many analysts are working for this company? SELECT COUNT(*) FROM EMP WHERE JOB = ANALYST ; COUNT(*) 2 6

7 Example 12 What are the minimum and maximum commissions paid to salesman, and what is the total amount of commission paid by the company? SELECT MIN(COMM), MAX(COMM), SUM(COMM) FROM EMP WHERE JOB = SALESMAN ; MIN(COMM) MAX(COMM) SUM(COMM) These functions ignore NULL values. NULL values are used to represent attributes whose values are missing, unknown or not applicable. In the case that all values in a column are NULL, these functions return a special NULL value. The GROUP BY Clause An additional helpful feature, used in connection with the group functions discussed above, is the GROUP BY clause. Using this clause, we can perform functions such as AVG, COUNT, MAX, MIN and SUM on subsets of rows in a table so far we have used them only with respect to all rows in a table. It is often helpful to think of the GROUP BY clause as grouping the rows of an intermediate result in the evaluation of the query. In Example 13, below, we have to group the rows in EMP according to their DEPTNO values. Example 13 How much salary is paid by each department? SELECT DEPTNO, SUM(SAL) FROM EMP GROUP BY DEPTNO; DEPTNO SUM(SAL) We note the following points about the GROUP BY clause. A GROUP BY clause may also consist of two or more column identifiers. If grouping is required on a column which contains NULL values then these NULL values form one group. In the output, NULL value groups are presented first, whether the required ordering is ASCENDING or DESCENDING. 7

8 The HAVING Clause Similar to the WHERE clause, which allows us to select particular rows, we can use the HAVING clause to select particular groups of rows. Only results for those groups of rows are shown, which satisfy the condition given in the HAVING clause of a query. Let s say we are interested in the average salary of departments where nobody earns less than In order to answer this query, it is necessary to determine the lowest salary of each department, and then to eliminate all departments which have employees with a salary below Then we have to calculate the average salary values for the remaining departments. Example 14 shows a query which does all this. Example 14 What are the average salaries in departments where the minimum salary is greater than 1000? SELECT DEPTNO, AVG(SAL) FROM EMP GROUP BY DEPTNO HAVINGMIN(SAL) > 1000; DEPTNO AVG(SAL) We have now extended the SELECT statement by two more clauses. The new general form of the SELECT statement is shown in Figure 1. SELECT FROM WHERE GROUP BY HAVING ORDER BY column-names table-names search-condition column-name group-select-condition column-names Figure 1: The form of a SELECT statement with WHERE, GROUP BY and ORDER BY clauses. Further Reading Elmasri and Navathe, chapter 8, section 8.2. Ramakrishnan and Gehrke, Database Management Systems, chapter 5, sections 1 3 and 5. Chris Walton (adapted from a note by Rob Procter) 8