Oracle Database Administration
Lecture 2
SQL language
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
• System control statements: alter
session
system
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)
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 ...]
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 ... ]
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)
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(*)
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
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
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
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)
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
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
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
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
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
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.
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
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
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)
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
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
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)
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
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
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
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
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
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
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
Sub-selects
• ORDER BY clause sub-select
SELECT
ID
FROM dept
ORDER BY (SELECT MAX (salary)
FROM employers
WHERE employers.dept = dept.ID)
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)
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 = ‘....’
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
Oracle extensions
• To get first 100 students in alphabetical order use the
following statement:
SELECT *
FROM (SELECT
FROM
ORDER BY
WHERE ROWNUM <=
*
students student
NAME DESC)
100
Oracle extensions
• Hierarchical queries: for table that references itself
• Special CONNECT BY clause
SELECT
FROM
CONNECT BY
START WITH
*
employers
boss_id = PRIOR id
id = 1;
• Selects all employers who’s boss (direct or not) is
employer with ID = 1
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;
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
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)