CSCU9Q5 Introduction to MySQL. Data Definition & Manipulation (Over ~two Lectures)

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1 CSCU9Q5 Introduction to MySQL Data Definition & Manipulation (Over ~two Lectures) 1

2 Contents Introduction to MySQL Create a table Specify keys and relations Empty and Drop tables 2

3 Introduction SQL is a declarative language for manipulating a relational database Use it to issue commands to the database for tasks such as: Creating and managing tables Inserting data into tables Searching for and retrieving data from tables Deleting data and tables MySQL is a particular version of SQL 3

4 Online Resource Is Oracle s MySQL documentation site note the 5.0 is the version we will use in practicals here. You will need: You will mostly need: Chapter 13: SQL Statement Syntax Chapter 11: Data Types The search facility is awful A better place to learn SQL is: 4

5 Database Engines MySQL supports a number of database engines, each designed for databases with different needs We will use the InnoDB engine as it supports inter-table constraints (Foreign keys) For more on Storage Engines, see: 5

6 Making MySQL Statements We tend to use UPPER CASE for reserved words Strings are enclosed in forward single quotes or double quotes Names of database elements such as tables and fields are enclosed in backwards `quotes` Statements are separated by semi-colons ; E.G SELECT `name` from `mytable` WHERE `name`= John You can drop the use of quotes if it is safe to do so, for example for names with no spaces or special characters. SELECT name from mytable WHERE name= John 6

7 SQL: CREATE TABLE The simplest form of CREATE TABLE looks like this: CREATE TABLE IF NOT EXISTS tablename (colname datatype,... ) CREATE TABLE Staff (Sno INT, Sname CHAR(20), Dept CHAR(20), Grade CHAR(7)) See for full details of CREATE TABLE 7

8 Data Types MySQL is strict about the use of data types. They include: VARCHAR variable length text strings INT integers DECIMAL numbers with decimal places DATE full date TEXT BOOL And others that you can read about here 8

9 Table Constraints The specification of a column can include some extras: default value (used if an insertion doesn't supply a value) and a column constraint (see below) And we can add table constraint(s) before the closing bracket. Both types of constraint are used to protect integrity (next slide) 9

10 SQL: CREATE TABLE: Data Integrity Column constraints: enforcing entity and referential integrity [NOT NULL NULL] [DEFAULT default_value] [AUTO_INCREMENT] [UNIQUE [KEY] [PRIMARY] KEY] [COMMENT 'string'] PRIMARY KEY (only one per table) FOREIGN KEY REFERENCES table (column) e.g. CREATE TABLE Staff (Sno INT PRIMARY KEY, Sname VARCHAR(20) NOT NULL, 10

11 SQL: CREATE TABLE: Data Integrity: Foreign Keys The last of these declares a foreign key. CREATE TABLE Staff ( Dept VARCHAR(20) FOREIGN KEY REFERENCES Depts (Dname), Grade VARCHAR(7) FOREIGN KEY REFERENCES Paytable ) The referenced column must be the key of its table We shall not be allowed to insert a row into Staff unless Dept and Grade contains a valid value (one found in the other table) or null we can miss out the bracketed column-name if it is the same in both tables 11

12 SQL: CREATE TABLE: Data integrity (continued): cascaded deletion We can add ON DELETE CASCADE to the REFERENCES part This means that if a row in the other table is deleted, all matching rows in this table should be deleted too For example, the Dependant table in the ER tutorial (for the dependants of employees) might declare the column Enum REFERENCES Employee ON DELETE CASCADE So if we delete employee 123 from the Employee table, then then all his/her dependants are deleted from the Dependant table, thus protecting referential integrity We should only do this for weak entities it would be a disaster if we deleted (say) student records as a result of deleting an Adviser of Studies 12

13 SQL: CREATE TABLE: Data integrity (continued): Deadly Embrace Notice that we cannot declare the Staff table until we have declared Depts and Paytable insert any data into Staff until we have matching data in Depts and Paytable This can form a deadly embrace suppose the Depts table references the Staff table (for Head of Dept, for example) then we cannot declare the Depts table until we have declared Staff ; and we cannot declare the Staff table until we have declared Depts the solution to this problem comes later 13

14 SQL: CREATE TABLE: Data integrity (continued) Table Constraints After the last field, we can add table-constraints these look like column-constraints, but they can reference more than one column CREATE TABLE HTR (Hour char(6), Teacher char(3), Room char(4), PRIMARY KEY (Hour, Teacher)) ; obviously, this is how to declare composite primary keys we can declare (possibly composite) foreign keys in the same sort of way, e.g. the Staff table could be rewritten to put the constraints at the end (next slide): 14

15 SQL: CREATE TABLE: Data Integrity (continued) Table Constraints (Continued) CREATE TABLE Staff (Sno INT PRIMARY KEY, Sname VARCHAR(20) NOT NULL, Dept VARCHAR(20) REFERENCES Depts (Dname), Grade VARCHAR(7) REFERENCES Paytable) ; could be written as CREATE TABLE Staff (Sno INT, Sname VARCHAR(20) NOT NULL, Dept VARCHAR(20), Grade VARCHAR(7), PRIMARY KEY(Sno), CONSTRAINT FOREIGN KEY (Dept) REFERENCES Depts (Dname), CONSTRAINT FOREIGN KEY (Grade) REFERENCES Paytable) 15

16 SQL: ALTERING AN EXISTING TABLE We can change tables, using ALTER TABLE, even after they contain data Amongst other possibilities, we can add or modify columns ALTER TABLE Staff ADD (StreetAddress VARCHAR(20), TownAddress VARCHAR(20)) ; ALTER TABLE Staff MODIFY (TownAddress DEFAULT 'Stirling') ; (we can enter staff before departments, then switch on the constraint) 16

17 SQL: ALTERING TABLE, continued Avoiding Deadly Embrace We can use ALTER TABLE to add a constraint This gets us out of the deadly embrace mentioned earlier CREATE TABLE Dept (Dname VARCHAR(20) PRIMARY KEY, Head INT) ; CREATE TABLE Staff (Sno INT PRIMARY KEY, Sname VARCHAR(20) NOT NULL Dept VARCHAR(20) CONSTRAINT DeptExist REFERENCES Dept (Dname) ALTER TABLE Dept ADD FOREIGN KEY (Head) REFERENCES Staff(Sno) 17

18 Dropping and Deleting We can completely remove a table: both its data (if any) and its definition DROP TABLE tablename ; DROP TABLE tablename CASCADE CONSTRAINTS ; the second form removes Foreign Key constraints in associated tables (which otherwise could not be updated) Removing the data alone (not the definition): DELETE FROM tablename ; DELETE FROM tablename WHERE condition ; we shall deal with conditions later 18

19 Getting data into tables There are two ways of using SQL to get data into tables Firstly, with the values in the SQL statement INSERT INTO Staff VALUES (123,'Lee','CompSci','II.7') ; if we are not loading all the columns, use this form: INSERT INTO Staff (Sno, Sname) VALUES (456, 'Waldenstein') ; Secondly, by extracting the data from existing tables INSERT INTO Loan SELECT DISTINCT Sno, Bno, Date_out FROM Staff_Borrower ; We can also use a bulk-loader utility (PHPMyAdmin has one) 19

20 Getting Even More Data In INSERT INTO `books` (`Name`,`Number`) VALUES ('book1', '1'), ('book2', '2'), ('book3', 3'); See for more details of the INSERT syntax 20

21 Changing Data in a Table Use UPDATE table SET field=value, field=value WHERE condition See for full syntax Next, we will look at manipulating table data Querying a database 21

22 CSC9UQ5 Introduction to MySQL DATA MANIPULATION 22

23 Getting Data From a Database with MySQL The SQL SELECT statement is used to retrieve data from one or more tables in a database It allows you to choose which row and which columns you would like to retrieve and allows joins across several tables. 23

24 Example Tables for This Lecture Books Borrowers Name Number Name Number Dept Book1 1 Anne 1 Maths Book2 2 Bill 2 Maths Book3 3 Claire 3 French Book4 4 Duncan 4 French Book5 5 Edward 5 French Loans BookNumber PersonNumber

25 SELECT From One Table SELECT * FROM Borrowers Shows whole table SELECT Name FROM Borrowers Lists Borrowers names only SELECT Number, Dept FROM Borrowers WHERE Name= Anne Get Anne s number and department 25

26 SELECT and Calculations There are many functions that you can include in a SELECT statement Some simple ones are: SELECT MAX(Number) FROM Borrowers Shows largest borrower number (Min is also available) SELECT Number+1 FROM Borrowers Adds 1 to each borrower number and reports it or even SELECT sin(45) Calculates the sine of 45 no need to reference a table at all 26

27 Joining Tables SELECT borrowers.name, books.name FROM borrowers, books Pairs every borrower with every book Not an awful lot of use Needs to be expanded to be useful 27

28 Looking up a Foreign Key Let s say we want to know who has borrowed Book1 The Loans table has this information, but not in a form that is immediately accessible: BookNumber PersonNumber SELECT Number FROM Books WHERE Name= Book1 Tells us that book1 has the ID number 1 SELECT PersonNumber FROM Loans WHERE BookNumber=1 Tells us that person 2 has the book SELECT Name FROM Borrowers WHERE Number=2 Tells us that person number 2 is Bill, so Bill has Book1 28

29 Put it all Together SELECT Borrowers.Name FROM Borrowers, Books, Loans WHERE Books.name="Book1" AND Books.Number=loans.booknumber AND Borrowers.number=loans.personnumber Returns Bill. Note the use of AND. You can also use OR and NOT. Other useful logic: >, <, <=, >=, =,!= (or <>) When comparing NULL, use IS NULL or IS NOT NULL You can search for partial strings using LIKE: SELECT Name FROM Borrowers WHERE Name LIKE "%e" The % character is a wild card, so above looks for anything ending in e SELECT Name FROM Borrowers WHERE Name LIKE _e" The _ is a single character wild card, so above looks for names with two characters ending in e 29

30 Comparison Examples SELECT * FROM table WHERE x BETWEEN 1 and 3 SELECT * FROM table WHERE x IN ( A, B, C ) SELECT * FROM table WHERE x NOT IN ( A, B, C ) Use brackets to enforce operator order: WHERE (a=1) AND (b=2 OR b=3)!= WHERE (a=1 AND b=2) OR (b=3) Note: Commands and Names (such as table or field names) are not case sensitive: Select = SELECT Table = table String literals may be case sensitive, depending on the collation. _ci at the end of the collation name means case insensitive Force a case sensitive search with SELECT * FROM table WHERE BINARY name="john" 30

31 How to Construct a SELECT Decide what columns you want in your results table: SELECT col1, col2 Then list all the tables involved in the selection (some won t actually provide a field in the select list, but are involved anyway SELECT col1, col2 FROM table1, table2, table3 Then follow the path from what you want to look up to the answer, building x=y statements joined by AND (or OR). SELECT col1, col2 FROM table1, table2, table3 WHERE table1.col1= target AND table2.col1=table1.col1 This is known as performing a Natural Join 31

32 More General Joins If we want to list all the people who have a book, and the books they have: SELECT Borrowers.Name, Books.name FROM Borrowers, Books, Loans WHERE Books.Number=loans.booknumber AND Borrowers.number=loans.personnumber Note that SELECT * FROM Borrowers, Books, Loans WHERE Books.Number=loans.booknumber AND Borrowers.number=loans.personnumber Is an Equijoin, and lists all the columns from all tables that meet the selection criteria, including them once for each table they appear in. This is not very useful, but shows that everything matches up. 32

33 Inner and Outer Joins We have just seen an inner join. Only rows that meet the criteria completely are returned. Now we look at outer joins where more data is returned about one or more of the fields. Lets say we want to list all of the people in the borrowers database, regardless of whether or not they have a book out. If they do have a book, we want to list that too, otherwise we will just return NULL for people with no current book For this, we need the JOIN command: 33

Join Syntax SELECT Borrowers.name, Books.name FROM Borrowers LEFT OUTER JOIN (Books, Loans) ON (Books.Number=loans.booknumber AND Borrowers.number=loans.

34 Join Syntax SELECT Borrowers.name, Books.name FROM Borrowers LEFT OUTER JOIN (Books, Loans) ON (Books.Number=loans.booknumber AND Borrowers.number=loans.personnumber) Note the syntax SELECT cols FROM table LEFT OUTER JOIN (table,table) ON (conditions) You can use similar syntax for an inner join: SELECT Borrowers.name, Books.name FROM Borrowers INNER JOIN (Books, Loans) ON (Books.Number=loans.booknumber AND Borrowers.number=loans.personnumber) Is the same as in previous example general join 34

35 A Contrived Example Who has the book with the lowest ID number? SELECT Borrowers.Name FROM Borrowers, Books, Loans WHERE Books.number = (SELECT MIN(number) FROM Books) AND Books.Number=loans.booknumber AND Borrowers.number=loans.personnumber The Answer, as we would expect, is Bill Note the use of brackets to enclose the sub-select and use of the function MIN() 35

36 Groups We may want to summarise the data in the database, and there are some statistical functions available. We have already seen MAX and MIN and there are others: AVG COUNT For example, how many people are there from each department? SELECT Dept, COUNT(Dept) FROM Borrowers GROUP BY Dept 36

37 How Many Books Does Each Department Have? We could show the list and count them ourselves: SELECT Borrowers.Dept FROM Borrowers, Books, Loans WHERE Books.Number=loans.booknumber AND Borrowers.number=loans.personnumber But SQL can do it for us: SELECT Dept, COUNT(Dept) FROM Borrowers, Books, Loans WHERE Books.Number=loans.booknumber AND Borrowers.number=loans.personnumber GROUP BY Dept You can select from the resultant table using HAVING SELECT Dept, COUNT(Dept) FROM Borrowers, Books, Loans WHERE Books.Number=loans.booknumber AND Borrowers.number=loans.personnumber GROUP BY Dept HAVING COUNT(Dept) > 2 37

38 Another Example SELECT Dept, COUNT(Dept) FROM Borrowers, Books, Loans WHERE Books.Number=loans.booknumber AND Borrowers.number=loans.personnumber GROUP BY Dept HAVING COUNT(Dept) = (SELECT MAX(COUNT(Dept))) Selects the department with the most books out and reports how many they have 38

39 Union Allows you to make more than one selection at the same time and put the results together A bit like using OR, but more flexible: SELECT Name FROM Borrowers WHERE Name="Anne" OR Name="Claire Is the same as SELECT Name FROM Borrowers WHERE Name="Anne UNION SELECT Name FROM Borrowers WHERE Name="Claire Which might seem pointless 39

40 Union But it is useful when selecting across more than one table SELECT Name FROM Borrowers UNION SELECT Name FROM Staff Selects the names of all borrowers and all staff Note that there is an Anne in both tables. UNION will only list Anne once. To see duplicate rows, use Staff Name Number Role Anne 1 Manager Fred 2 Engineer George 3 Engineer Harry 4 Engineer UNION ALL 40

41 Selection Manipulation Here are a few things you can do to the selected list: Sort the results by one or more field ORDER BY field, field, [DESC] Force a query to contain unique entries only: SELECT DISTINCT Select a given number of entries starting at a given offset SELECT * FROM table LIMIT(offset,count) 41

42 More on Sub Queries We can use a sub query, as we have already seen, but what happens when the sub query produces more than 1 row? SELECT * FROM staff WHERE Name = ANY (SELECT Name FROM Borrowers) Selects staff whose name appears in both Staff and Borrowers tables SELECT * FROM borrowers WHERE Number > ALL (SELECT Number FROM staff) Selects those borrowers who have a higher number than ALL of the staff 42

43 Aliases You can give an alias to a table or column to make it easier to refer to later in a query or to make it easier to read in the output: SELECT AVG(Number) FROM books shows AVG(Number) 3 SELECT AVG(Number) Average FROM books shows Average 3 43

44 Aliases SELECT Role, COUNT(Role) FROM staff GROUP BY role HAVING COUNT(role) > 1 Can be re-written as SELECT Role, COUNT(Role) count FROM staff GROUP BY role HAVING count > 1 Where count is an alias for COUNT(Role) You can also do this sort of thing: SELECT * FROM Books a, Borrowers b WHERE a.number = b.number We have made aliases for the tables to make reference to them easier 44

COSC 304 Introduction to Database Systems SQL DDL. Dr. Ramon Lawrence University of British Columbia Okanagan

COSC 304 Introduction to Database Systems SQL DDL Dr. Ramon Lawrence University of British Columbia Okanagan ramon.lawrence@ubc.ca SQL Overview Structured Query Language or SQL is the standard query language