Selections. Lecture 4 Sections Robb T. Koether. Hampden-Sydney College. Wed, Jan 22, 2014

Transcription

1 Selections Lecture 4 Sections Robb T. Koether Hampden-Sydney College Wed, Jan 22, 2014 Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

2 1 Datatypes 2 Constraints 3 Storage Engines 4 Selections The SELECT Statement Renaming Attributes The WHERE Clause The ORDER BY Clause 5 Assignment Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

3 Outline 1 Datatypes 2 Constraints 3 Storage Engines 4 Selections The SELECT Statement Renaming Attributes The WHERE Clause The ORDER BY Clause 5 Assignment Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

4 Date and Time Types Standard date and time types DATE - A date in the format YYYY-MM-DD. TIME - A time of day in the format HH:MM:SS. DATETIME - A date and time in the format YYYY-MM-DD HH:MM:SS. TIMESTAMP - A date and time in the format YYYY-MM-DD HH:MM:SS. YEAR(n) - A year as either YYYY when n = 4 or YY when n = 2. Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

5 Date and Time Types The values of a DATE may range from to The values of a DATETIME may range from :00:00 to :59:59. The values of a TIMESTAMP may range from :00:01 to :14:07. A TIMESTAMP attribute may be assigned the value CURRENT_TIMESTAMP or NOW(). Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

6 DATETIME Format There are several formats available for date input. Format Example YYYY-MM-DD YYYY-M-DD YYYY-MM-D YYYY-M-D YY-MM-DD YYYY/MM/DD 2013/01/05 YY/MM/DD 13/01/05 YYYY:MM:DD 2013:01:05 YYYYMMDD YYMMDD YYYY 2013 YY 13 Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

7 DATETIME Format If the year is given as a four-digit number YYYY, it may be in the range 1901 to If the year is given as a two-digit number YY, then represent represent Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

8 Outline 1 Datatypes 2 Constraints 3 Storage Engines 4 Selections The SELECT Statement Renaming Attributes The WHERE Clause The ORDER BY Clause 5 Assignment Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

9 Default Values Default Values ssn CHAR(11) DEFAULT ; We may use the DEFAULT clause to specify a default value for an attribute when a value is not given. For example, in the employees relation, if the Social Security number is not given, then we may let be the default value. Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

10 Null Values The NOT NULL Modifier lname CHAR(20) NOT NULL; If we designate no default value and do not assign a value to an attribute in a tuple, then that attribute is given the value NULL. We may specify that a value may not be NULL. This prevents the value from being assigned NULL or being changed to NULL. For example, in the employees relation, we may specify that the last name be non-null. Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

11 The CHECK Clause The CHECK Clause dept TINYINT CHECK(dept >= 1 AND dept <= 5); Even though the value of the attribute is of the right type, we may want to check that it is in the right range. For example, in the employees relation, the department number must be between 1 and 5. This feature is currently not available in MySQL. Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

12 Primary Keys The PRIMARY KEY Clause PRIMARY KEY (ssn, dep_name) Every table should have a primary key. The primary key may be a single attribute or a set of attributes. A table s primary key must be unique for each tuple. For example, in the dependents relation, the Social Security number and name together must be unique. That would be a natural choice for the primary key. Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

13 Unique Values The UNIQUE Modifier dept_name VARCHAR(64) UNIQUE; We may use the UNIQUE modifier to specify that a given attribute will have a unique value for each tuple, even though it may not be a primary key. For example, in the departments relation, we may assume that each department has a unique name. Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

14 Outline 1 Datatypes 2 Constraints 3 Storage Engines 4 Selections The SELECT Statement Renaming Attributes The WHERE Clause The ORDER BY Clause 5 Assignment Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

15 Storage Engines MySQL uses a storage engine to process queries. There are two main storage engines for MySQL. MyISAM (My Indexed Sequential Access Method) innodb (Derived from Innobase, a Finnish company) The features available in MySQL and their performance depend on the storage engine. Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

16 MyISAM Advantages of MyISAM over innodb Easier to design databases. Faster execution. Especially fast at reading tables. Advantages of innodb over MyISAM Supports data integrity constraints. Supports transactions. Faster for inserting data and updating tables. Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

17 Designating the Storage Engine Designating the Storage Engine CREATE TABLE table_name (attribute_list) ENGINE = engine_name; When creating a table, after describing its attributes, specify the storage engine. If the storage engine is omitted, then the default is MyISAM. Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

18 The employees Table The employees Table CREATE TABLE employees ( fname char(20) DEFAULT NULL, lname char(20) NOT NULL, ssn char(11) NOT NULL DEFAULT , bdate date DEFAULT NULL, sex char(1) DEFAULT NULL, salary decimal(10,2) DEFAULT NULL, dept int(11) DEFAULT NULL, PRIMARY KEY (ssn) ) ENGINE=InnoDB DEFAULT CHARSET=latin1 Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

19 Outline 1 Datatypes 2 Constraints 3 Storage Engines 4 Selections The SELECT Statement Renaming Attributes The WHERE Clause The ORDER BY Clause 5 Assignment Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

20 Outline 1 Datatypes 2 Constraints 3 Storage Engines 4 Selections The SELECT Statement Renaming Attributes The WHERE Clause The ORDER BY Clause 5 Assignment Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

21 Selections The SELECT Statement SELECT attribute_list FROM table_name WHERE condition ORDER BY attribute_list; The SELECT command will select tuples from a relation. The attribute list can be *, which means all attributes. The WHERE and ORDER BY clauses are optional. Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

22 Selections SELECT Example SELECT * FROM employees; fname lname ssn bdate sex salary dept Alice Smith F Barbara Brown F James Green M Jennifer Wallace F Frank Gilbert M Joyce English F Richard Johnson M Ernest Roth M Susan Lane F John Kohler M Raymond Jones M Amy Ford F Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

23 Selections SELECT Example SELECT fname, lname FROM employees; fname lname Alice Smith Barbara Brown James Green Jennifer Wallace Frank Gilbert Joyce English Richard Johnson Ernest Roth Susan Lane John Kohler Raymond Jones Amy Ford We can specify only those attributes that we want to see. Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

24 Selections SELECT Example SELECT CONCAT(fname,, lname) FROM employees; CONCAT(fname,, lname) Alice Smith Barbara Brown James Green Jennifer Wallace Frank Gilbert Joyce English Richard Johnson Ernest Roth Susan Lane John Kohler Raymond Jones Amy Ford In the attribute list, we may write expressions as well as simple attributes. Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

25 Outline 1 Datatypes 2 Constraints 3 Storage Engines 4 Selections The SELECT Statement Renaming Attributes The WHERE Clause The ORDER BY Clause 5 Assignment Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

26 Renaming Attributes SELECT Example SELECT CONCAT(fname,, lname) AS emp_name FROM employees; emp_name Alice Smith Barbara Brown James Green Jennifer Wallace Frank Gilbert Joyce English Richard Johnson Ernest Roth Susan Lane John Kohler Raymond Jones Amy Ford Using the AS clause, we can rename the attribute. Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

27 Outline 1 Datatypes 2 Constraints 3 Storage Engines 4 Selections The SELECT Statement Renaming Attributes The WHERE Clause The ORDER BY Clause 5 Assignment Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

28 The WHERE Clause WHERE Example SELECT fname, lname, salary FROM employees WHERE salary > 40000; fname lname salary James Green Jennifer Wallace Ernest Roth Susan Lane Raymond Jones In the WHERE clause, we write a boolean expression that will filter out some of the tuples. Use AND, OR, and NOT for compound expressions. Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

29 The WHERE Clause WHERE Example SELECT fname, lname, sex, bdate, salary FROM employees WHERE (bdate >= AND sex = M ) OR (bdate >= AND sex = F ); fname lname sex bdate salary Barbara Brown F Jennifer Wallace F Joyce English F Ernest Roth M Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

30 The WHERE Clause WHERE Example SELECT fname, lname, sex, bdate, salary FROM employees WHERE NOT((bdate >= AND sex = M ) OR (bdate >= AND sex = F )); fname lname sex bdate salary Alice Smith F James Green M Frank Gilbert M Richard Johnson M Susan Lane F John Kohler M Raymond Jones M Amy Ford F Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

31 The WHERE Clause WHERE Example SELECT fname, lname, sex, bdate, salary FROM employees WHERE (bdate < OR sex = F ) AND (bdate < OR sex = M ); fname lname sex bdate salary Alice Smith F James Green M Frank Gilbert M Richard Johnson M Susan Lane F John Kohler M Raymond Jones M Amy Ford F Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

32 Outline 1 Datatypes 2 Constraints 3 Storage Engines 4 Selections The SELECT Statement Renaming Attributes The WHERE Clause The ORDER BY Clause 5 Assignment Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

33 The ORDER BY Clause ORDER BY Example SELECT fname, lname, salary FROM employees ORDER BY salary; fname lname salary Joyce English Amy Ford Frank Gilbert Richard Johnson Alice Smith Barbara Brown John Kohler Jennifer Wallace Ernest Roth Susan Lane Raymond Jones James Green We can use the ORDER BY clause to sort the tuples by specified fields. Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

34 The ORDER BY Clause ORDER BY Example SELECT fname, lname, salary FROM employees WHERE sex = F ORDER BY salary; fname lname salary Joyce English Amy Ford Alice Smith Barbara Brown Jennifer Wallace Susan Lane The ORDER BY clause comes after the WHERE clause. Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

35 The ORDER BY Clause ORDER BY Example SELECT fname, lname, sex FROM employees ORDER BY sex, lname; fname lname sex Barbara Brown F Joyce English F Amy Ford F Susan Lane F Alice Smith F Jennifer Wallace F Frank Gilbert M James Green M Richard Johnson M Raymond Jones M John Kohler M Ernest Roth M If we specify more than one attribute, then they are treated as major and minor sort fields. Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

36 The ORDER BY Clause ORDER BY Example SELECT fname, lname, sex, salary FROM employees ORDER BY sex, salary DESC; fname lname sex salary Susan Lane F Jennifer Wallace F Barbara Brown F Alice Smith F Amy Ford F Joyce English F James Green M Raymond Jones M Ernest Roth M John Kohler M Richard Johnson M Frank Gilbert M We can specify that any sort field be sorted in descending order. Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

37 Outline 1 Datatypes 2 Constraints 3 Storage Engines 4 Selections The SELECT Statement Renaming Attributes The WHERE Clause The ORDER BY Clause 5 Assignment Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38

38 Assignment Assignment Read Sections Robb T. Koether (Hampden-Sydney College) Selections Wed, Jan 22, / 38