1. Oracle Database Administration Lecture 2 SQL language

2. Types of SQL statements DDL (Data Definition Language) : create, drop, alter, for example : CREATE TABLE DML (Data Modification Language): select, update, delete, insert Transaction control statements: commit, rollback, savepoint Session control statements: alter session System control statements: alter system

3. DML Statements: SELECT SELECT –query data, –does not modify tables –does not lock (unless FOR UPDATE ) SELECT * FROM users WHERE id = 1 FOR UPDATE –does not wait for any locks (unless FOR UPDATE )

4. DML Statements: UPDATE UPDATE –update data in one table (any number of rows) –locks updated data (until transaction end) –waits for locks if data is locked (can cause deadlock) –syntax: UPDATE table SET col1 = value1, col2 = value2 [ WHERE...]

5. DML Statements: DELETE DELETE –deletes rows from one table (any number of rows) –locks deleted data (update will wait) –waits for locks if data is locked (can cause deadlock) –syntax: DELETE FROM table [ WHERE... ]

6. DML Statements: INSERT INSERT –inserts one row or multiple rows –can sometimes wait for a lock Syntax: INSERT INTO table (col1, col2) VALUES (val1, val2) or INSERT INTO table VALUES (val1, val2) or INSERT INTO table (col1, col2) SELECT val1, val2 FROM... (regular select statement)

7. SELECT – basic syntax Simple version: SELECT column_list FROM table [WHERE condition] Example with table alias: SELECT u.last_name, u.first_name FROM users u – u is an alias for table users Column alias: SELECT count(*) AS cnt FROM users cnt is column alias for function count(*)

8. SELECT – all columns Select all columns from a table using “*” The following statements are identical: – SELECT * FROM users – SELECT users.* FROM users – SELECT u.* FROM users u To select all columns and more, use the following syntax: SELECT u.*, UPPER(last_name) FROM users

9. SELECT – other user tables To select data from some other user: SELECT users.first_name, users.last_name FROM user_name.users Note: user_name cannot be used in column list, the following is illegal: SELECT user_name.table1.col1, user_name.table1.col2 FROM user_name.table1 To select data from other user’s table current must have one of the following: SELECT privilege on the table SELECT ANY TABLE system privilige

10. WHERE condition WHERE clause specifies what records to include There can be any number of conditions combined with AND, OR, NOT There also can be conditions: – EXISTS (subquery), NOT EXISTS (subquery) – value IN (subquery), value NOT IN (subquery) SQL functions can be used: SELECT * FROM users WHERE hire_date > SYSDATE – 10 SELECT * FROM users WHERE LENGTH(last_name) > 20

11. WHERE condition examples SELECT * FROM departments WHERE EXISTS(SELECT * FROM employees WHERE employees.dept_id = departments.id) SELECT * FROM departments WHERE id IN (SELECT dept_id FROM employees)

12. SQL Joins Joining tables – selecting data from two or more tables at the same time. Types of joins: –Inner joins – typical –Outer joins –Cross joins – rarely used, sometimes used by accident by writing incorrect SELECT statement

13. INNER JOIN Explicit syntax: use INNER JOIN keywords Implicit syntax: –provide more than one table in FROM clause –provide join condition (in WHERE clause) –Joins without join condition produce Cross join

14. Joins Simple explicit join: SELECT student.id, group.id, student.name, group.name FROM student INNER JOIN group ON student.group_id = group.id Implicit join: SELECT student.id, group.id, student.name, group.name FROM student, group WHERE student.group_id = group.id

15. Cross join Explicit notation: SELECT student.id, group.id, student.name, group.name FROM student CROSS JOIN group Implicit notation: SELECT student.id, group.id, student.name, group.name FROM student, group in both cases: number of rows returned = number of students * number of groups

16. Complex join select Any number of tables can appear in SELECT statement, example: SELECT student.id, group.id, course.id, grade.grade student.name, group.name FROM student, group, course, grade, student_courses courses WHERE student.group_id = group.id AND courses.student_id = student.id AND courses.course_id = course.id AND grade.course_id = course.id AND student.id = grade.student_id

17. Inner Joins Statement: SELECT student.id, group.id, student.name, group.name FROM student, group WHERE student.group_id = group.id does not return empty groups.

18. Outer joins To return records with no matching join condition use outer join statement: SELECT student.id, group.id, student.name, group.name FROM student LEFT OUTER JOIN group ON student.group_id = group.id Left outer join always returns results from the “left” table (student), even if there are no matching rows in the right table (group) If there is no matching group: group.id IS NULL and group.name IS NULL

19. Outer joins RIGHT OUTER JOIN always returns results from the right table even if there are no matching rows in the left table FULL OUTER JOIN returns results from both tables

20. Oracle syntax for outer joins Statement: SELECT student.id, group.id, student.name, group.name FROM student, group WHERE student.group_id (+) = group.id returns one record for each student and one record for empty group (student.id and student.name are NULL)

21. Outer join (+) operator can be on either side of the = operator: SELECT student.id, group.id, student.name, group.name FROM student, group WHERE student.group_id = group.id (+) returns all students including ones with no group (+) operator cannot be on both sides of the = operator: WHERE student.group_id (+) = group.id (+) is illegal

22. Self join Table can be joined to itself – self join. One of the tables must be aliased in the FROM clause: SELECT dept.id, parent_dept.id, FROM dept, dept as parent_dept WHERE dept.parent_id = parent_dept.id

23. Group functions Select statement can include group function(s) in the column clause: SELECT max(salary), min(salary) FROM employers WHERE employers.dept = 2 Select statement with no GROUP BY clause return one record Group functions: count, sum avg, min, max stddev, variance (Oracle specific)

24. Group functions Statement with group function and no GROUP BY clause cannot have “regular” columns in SELECT list. The following is illegal: SELECT max(salary), min(salary), name FROM employers WHERE employers.dept = 2

25. Group by clause Records can be grouped using specified columns: SELECT max(salary), min(salary), dept FROM employers WHERE dept != 2 GROUP BY dept Select clause can only include: group functions columns from the GROUP BY clause

26. Having clause HAVING clause is used with GROUP BY statement HAVING clause filters records returned after grouping WHERE clause filters records before grouping SELECT max(salary), min(salary), dept FROM employers WHERE dept != 2 GROUP BY dept HAVING min(salary) > 100

27. UNION UNION, UNION ALL, MINUS, INTERSECT combine results of two independent SELECT statements Both statements must return the same number of columns with the same data types Column names are taken from the first SELECT statement MINUS and INTERSECT statements can be resource consuming SELECT student.name FROM student UNION ALL SELECT teacher.name FROM teacher

28. ORDER BY ORDER BY specifies how records are sorted ORDER BY clause can include: column name or column index (1-based) sort order (ASC or DESC) SELECT name, salary FROM employers ORDER BY salary DESC, 1 ASC SELECT name as personName, salary FROM employers ORDER BY salary DESC, personName ASC

29. Sub-selects SELECT name, salary FROM employers WHERE salary = (SELECT MIN(salary) FROM employers) In Oracle sub selects can be used in: WHERE clause FROM clause SELECT clause ORDER BY clause

30. Sub-selects FROM clause sub-select SELECT COUNT(*) FROM (SELECT MAX(salary) FROM employers) SELECT clause SELECT ID, (SELECT MAX (salary) FROM employers WHERE employers.ID = dept.ID) FROM dept

31. Sub-selects ORDER BY clause sub-select SELECT ID FROM dept ORDER BY (SELECT MAX (salary) FROM employers WHERE employers.dept = dept.ID)

32. WITH clause WITH clause lets you assign a name to a subquery block. The query can be then referenced in many places by specifying query name WITH w AS ( SELECT id FROM emp WHERE emp.status = 'ACTIVE') SELECT * FROM w emp1 WHERE EXISTS (SELECT * FROM w emp2 WHERE emp1.id = emp2.boss_id)

33. Oracle extensions ROWID pseudo column – unique identifier of a row in entire database. Will not change when the row is updated SELECT ROWID, ID FROM student....... DELETE FROM student WHERE ROWID = ‘....’

34. Oracle extensions ROWNUM pseudo column – numbers rows (starting from 1) that are returned in a query. Can be used to limit the number of rows returned: SELECT * FROM student WHERE ROWNUM <= 100 returns at most 100 rows DELETE FROM student WHERE ROWNUM <= 100 deletes at most 100 rows ROWNUMs are assigned before the rows are ordered SELECT * FROM student WHERE ROWNUM <= 100 ORDER BY student.name DESC

35. Oracle extensions To get first 100 students in alphabetical order use the following statement: SELECT * FROM (SELECT * FROM students student ORDER BY NAME DESC) WHERE ROWNUM <= 100

36. Oracle extensions Hierarchical queries: for table that references itself Special CONNECT BY clause SELECT * FROM employers CONNECT BY boss_id = PRIOR id START WITH id = 1; Selects all employers who’s boss (direct or not) is employer with ID = 1

37. Hierarchical queries Special LEVEL pseudo-column is available in hierarchical queries: SELECT level, e.* FROM employers e CONNECT BY e.boss_id = PRIOR e.id START WITH e.id = 1;

38. Practice SQL statements create table employees ( id number, name varchar2(100), boss_id number, dept_id number, salary number ); create table departments ( id number, name varchar2(100)); Select total number of employees Select total salary paid to all employees Select department name, total salary, average salary, number of employees in each department (including departments with no employees) Select employees with highest salary in each department Select employees that earn more than their direct boss

39. Practice SQL statements Select employees that earn more than their direct or indirect boss Select department where works the employee with the lowest salary Select five employees with the highest salaries Select employees from 6 - 10 on the salary list (from 6th to 10th person sorting by salary) (!) Select employers that earn more than average in their department Select departments with average salary higher than total average salary in the company Select employees that have more than 5 subordinates (direct or indirect)