CS 464/564 Introduction to Database Management System Instructor: Abdullah Mueen

Transcription

1 CS 464/564 Introduction to Database Management System Instructor: Abdullah Mueen LECTURE 9: INTRODUCTION TO SQL SELECT-FROM-WHERE STATEMENTS SUBQUERIES

2 Set-up the database 1. Log in to your machine using your CS username and password 2. Press Win Key and type "Terminal" 3. Press Enter on terminal Icon to open Linux Terminal 4. Run this commands in linux Terminal without the quotation mark: 4.1. "wget Wait for download to finish "ls" You should see "employees_db.tar.gz" in the list "tar -xvzf employees_db.tar.gz" 4.4. "cd employees_db"

3 Set-up the database 5. Run the following commands in the terminal to log in to your MySQL account (You should use your MySQL username and password in all the next steps): 5.1. "mysql -h chester.cs.unm.edu -u username -p" 5.2. Enter your password If you see "mysql>" in the terminal (this is called mysql prompt), it means you have logged in to your MySQL account successfully Type show databases; and press Enter. You should see 2 database names. The first one is "information_schema" and the second one is the same as your MySQL username (Let's call it MyUsername). You will use "MyUsername" database for all of your tasks in this course "use MyUsername;" 5.5. "source employees.sql" Wait for data insertion process to finish 5.6. "show tables;" You should see 6 tables in the list. How to log in to mysql server without entering password everytime

4 Why SQL? SQL is a very-high-level language. Say what to do rather than how to do it. Avoid a lot of data-manipulation details needed in procedural languages like C++ or Java. Database management system figures out best way to execute query. Called query optimization. MySQL MANUAL 4

5 Select-From-Where Statements SELECT desired attributes FROM one or more tables WHERE condition about tuples of the tables 5

6 Our Running Example All our SQL queries will be based on the following database schema. Underline indicates key attributes. 6

7 Example Using employees(emp_no, birth_date, first_name, last_name, gender, hire_date), what are names of the male employees who are born after 1993? FROM employees WHERE Gender = M AND year(birth_date) > 1993; Notice SQL uses single-quotes for strings. SQL is case-insensitive, except inside strings. 7

8 Meaning of Single-Relation Query Begin with the relation in the FROM clause. Apply the selection indicated by the WHERE clause. Apply the extended projection indicated by the SELECT clause. 8

9 Operational Semantics To implement this algorithm think of a tuple variable (tv) ranging over each tuple of the relation mentioned in FROM. Check if the current tuple satisfies the WHERE clause. If so, compute the attributes or expressions of the SELECT clause using the components of this tuple. 9

10 * In SELECT clauses When there is one relation in the FROM clause, * in the SELECT clause stands for all attributes of this relation. Example using departments(dept_no, dept_name): SELECT * FROM departments; 10

11 Renaming Attributes If you want the result to have different attribute names, use AS <new name> to rename an attribute. Example based on departments(dept_no, dept_name): SELECT dept_name AS division FROM departments; 11

12 Expressions in SELECT Clauses Any expression that makes sense can appear as an element of a SELECT clause. Example: from salaries(emp_no, salary, from_date, to_date): SELECT emp_no, salary/12 AS monthlysalary FROM salaries WEHRE monthlysalary > 12000; 12

13 Another Example: Constant Expressions From departments(dept_no, dept_name): SELECT dept_name, works hard AS worklevel FROM departments WHERE dept_name = xxxx ; 13

14 Complex Conditions in WHERE Clause From employees(emp_no, birth_date, first_name, last_name, gender, hire_date), find the male employees that who was hired in 2013: SELECT first_name, hire_date FROM employees WHERE gender = M AND year(hire_date) = 2013; 14

15 Patterns WHERE clauses can have conditions in which a string is compared with a pattern, to see if it matches. General form: NOT LIKE <pattern> <Attribute> LIKE <pattern> or <Attribute> Pattern is a quoted string with % = any string ; _ = any character. 15

16 Example From employees(emp_no, birth_date, first_name, last_name, gender, hire_date) find the consumers with exchange 555: SELECT first_name FROM employees WHERE last_name LIKE %---son ; 16

17 NULL Values Tuples in SQL relations can have NULL as a value for one or more components. Meaning depends on context. Two common cases: Missing value : e.g., we know 7-11 has some address, but we don t know what it is. Inapplicable : e.g., the value of attribute spouse for an unmarried person. 17

18 Comparing NULL s to Values The logic of conditions in SQL is really 3-valued logic: TRUE, FALSE, UNKNOWN. When any value is compared with NULL, the truth value is UNKNOWN. But a query only produces a tuple in the answer if its truth value for the WHERE clause is TRUE (not FALSE or UNKNOWN). 18

19 Three-Valued Logic To understand how AND, OR, and NOT work in 3-valued logic, think of TRUE = 1, FALSE = 0, and UNKNOWN = ½. AND = MIN; OR = MAX, NOT(x) = 1-x. Example: TRUE AND (FALSE OR NOT(UNKNOWN)) = MIN(1, MAX(0, (1 - ½ ))) = MIN(1, MAX(0, ½ ) = MIN(1, ½ ) = ½ = UNKNOWN. 19

20 Surprising Example From the following Sells relation: store candy price 7-11 Twizzler NULL SELECT store FROM Sells WHERE price < 2.00 OR price >= 2.00; UNKNOWN UNKNOWN UNKNOWN 20

21 Reason: 2-Valued Laws!= 3-Valued Laws Some common laws, like commutativity of AND, hold in 3-valued logic. But not others, e.g., the law of the excluded middle : p OR NOT p = TRUE. When p = UNKNOWN, the left side is MAX( ½, (1 ½ )) = ½!= 1. 21

22 Subqueries A parenthesized SELECT-FROM-WHERE statement (subquery ) can be used as a value in a number of places, including FROM and WHERE clauses. Example: in place of a relation in the FROM clause, we can place another query, and then query its result. Can use a tuple-variable to name tuples of the result. 22

23 Subqueries That Return One Tuple If a subquery is guaranteed to produce one tuple, then the subquery can be used as a value. Usually, the tuple has one component. A run-time error occurs if there is no tuple or more than one tuple. 23

24 Example employees(emp_no, birth_date, first_name, last_name, gender, hire_date), find the employees that were hired the same day as emp_no, 500. Two queries would surely work: Find the hire_date for emp_no 500. Find the employees that have the hire_date same as above. 24

25 Query + Subquery Solution SELECT first_name FROM employees WHERE hire_date = (SELECT hire_date FROM employees WHERE emp_no=500); 25

26 The IN Operator <tuple> IN <relation> is true if and only if the tuple is a member of the relation. <tuple> NOT IN <relation> means the opposite. IN-expressions can appear in WHERE clauses. The <relation> is often a subquery. 26

27 Example From employees(emp_no, birth_date, first_name, last_name, gender, hire_date) and dept_manager(dept_no, emp_no, from_date, to_date), find the name of the department managers. SELECT first_name, last_name FROM employees WHERE emp_no IN (SELECT emp_no FROM dept_manager ); 27

28 The Exists Operator EXISTS( <relation> ) is true if and only if the <relation> is not empty. Example: From employees(emp_no, birth_date, first_name, last_name, gender, hire_date), find those employees that were hired with some other employees. 28

29 Example Query with EXISTS SELECT first_name FROM employees e1 WHERE EXISTS( SELECT * FROM employees WHERE hire_date = e1.hire_date); Notice scope rule: hire_date refers to closest nested FROM with a relation having that attribute. 29

30 The Operator ANY x = ANY( <relation> ) is a boolean condition true if x equals at least one tuple in the relation. Similarly, = can be replaced by any of the comparison operators. Example: x > ANY( <relation> ) means x is not the smallest tuple in the relation. Note tuples must have one component only. 30

31 The Operator ALL Similarly, x <> ALL( <relation> ) is true if and only if for every tuple t in the relation, x is not equal to t. That is, x is not a member of the relation. The <> can be replaced by any comparison operator. Example: x >= ALL( <relation> ) means there is no tuple larger than x in the relation. 31

32 Example From salaries(emp_no, salary, from_date, to_date), find the candies sold for the highest price. SELECT emp_no FROM salaries WHERE salary >= ALL( SELECT salary FROM salaries); 32