COMP 244 DATABASE CONCEPTS & APPLICATIONS

Transcription

1 COMP 244 DATABASE CONCEPTS & APPLICATIONS Querying Relational Data 1

2 Querying Relational Data A query is a question about the data and the answer is a new relation containing the result. SQL is the most popular language for writing queries. 2

3 Format of SQL Queries Queries should take the following form: SELECT FROM WHERE [ DISTINCT ] select-list from-list qualification The WHERE clause is optional Logical connectives (AND, OR, and NOT) can be used in the WHERE clause along with comparison operators (<, <=, = <>, >=, >) 3

4 SELECT * FROM Students; This says to make the result relation contains all the columns in the original Students relation This will generate the original table with all columns and all rows Students sid name age gpa sid name age gpa Khalsa Khalsa Ahmed Ahmed Muna Muna Fatma Fatma Rashid Rashid

5 SELECT name, age FROM Students; This will generate the original table with only the name and age columns and all rows We can display only specific columns from table Students Students sid name age gpa name age Khalsa 2.9 Khalsa Ahmed Ahmed Muna 2.5 Muna Fatma Fatma Rashid Rashid 19 5

6 We can use the WHERE clause to limit the number of rows displayed SELECT * FROM Students WHERE gpa < 3.0; This says to include only rows in which gpa < 3.0 Students sid name age gpa sid name age gpa Khalsa Khalsa Ahmed Muna Muna Fatma Rashid

7 Expressions and Aggregates The select-list can contain expressions Example: The following SQL statement will convert the student s GPA to a percentage. SELECT (gpa/4.0)*100 FROM Students; 7

8 Expressions and Aggregates The select-list also can contain aggregate functions (SUM, COUNT, AVG, MAX, MIN, etc.) Example: The following SQL statement will return the total number of students. SELECT FROM count(*) Students; 8

9 Expressions and Aggregates SQL supports many aggregate functions, such as COUNT, SUM, AVG, MAX, MIN Example: Find the highest gpa of all students that are under 20 years of age: SELECT MAX(S.gpa) FROM Students S WHERE S.age < 20 Example: Find all students that are on probation: SELECT COUNT(*) FROM Students S WHERE S.gpa < 2.0 9

10 Expressions and Aggregates Example: Find the youngest students in the university: The following query is illegal in SQL: SELECT FROM S.name, MIN(S.age) Students S If the SELECT clause contains an aggregate function, then it must use all aggregate functions unless the query contains a GROUP BY clause 10

11 Expressions and Aggregates One can give a name for each expression or aggregate Examples: SELECT (gpa/4.0)*100 AS percentage FROM Students; SELECT count(*) AS enrolled FROM Enrollments; 11

12 Date and time expressions MySQL supports many date and time functions. Examples: SELECT day(dob) AS day month(dob) AS month year(dob) AS year FROM Students; 12

13 MySQL allows sorting of information either in ascending (ASC) or descending (DESC) order. Example: Sort Orders SELECT name, gpa FROM Students ORDER BY name; 13

14 Sort Orders Another Sorting Example: The following SQL statement will first sort students in descending order by gpa. Then, it will sort each group of students that are having the same gpa by name. SELECT name, gpa FROM Students ORDER BY gpa DESC, name ASC; 14

15 Pattern Matching MySQL supports the use of pattern matching using the LIKE operator and wildcards (% and _) Example: Find all students whose name starts with A and ends with D SELECT S.name FROM Students S WHERE S.name LIKE A%D ; 15

16 GROUP BY and HAVING Clauses Often, we need to apply aggregate functions to groups of rows and not to the entire relation Example: Find the average gpa of each age group SELECT FROM age, AVG(gpa) AS Average_GPA Students GROUP BY age; Note: The select-list can only contain an aggregate function or a column that appears in the GROUP BY clause. 16

17 GROUP BY and HAVING Clauses Example: Find the number of students that received each letter grade in COMP244: SELECT grade, COUNT(*) AS Count FROM Enrollments WHERE cid = COMP244 GROUP BY grade; 17

18 GROUP BY and HAVING Clauses Example: Find the average gpa of each age group having at least two students per group: SELECT age, AVG(gpa) AS Average_GPA FROM Students GROUP BY age HAVING COUNT(*) > 1; 18

19 GROUP BY and HAVING Clauses Example: Find the average gpa of each age group having at least two students per group and gpa of each group >= 2.0: SELECT age, AVG(gpa) AS Average_GPA FROM Students GROUP BY age HAVING COUNT(*) > 1 AND EVERY (gpa >= 2.0); 19

20 GROUP BY and HAVING Clauses Note: The previous example is not the same as the following: SELECT age, AVG(gpa) AS Average_GPA FROM Students WHERE gpa >= 2.0 GROUP BY age HAVING COUNT(*) > 1; 20

21 NULL Values Unknown values are stored as NULL values One can use the IS NULL operator to check for null values All aggregate functions, except COUNT, discard null values in their calculations Example: Find all students with no assigned advisor SELECT FROM WHERE id, name Students advisor IS NULL; 21