Oracle Database Administration Lecture 2 SQL language.

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 session• System control statements: alter 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 examplesSELECT * 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.idLeft 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 groupingSELECT 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-selectsSELECT 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-selectSELECT COUNT(*) FROM

(SELECT MAX(salary) FROM employers)

• SELECT clauseSELECT ID, (SELECT MAX (salary)

FROM employers

WHERE employers.ID = dept.ID)

FROM dept

Sub-selects• ORDER BY clause sub-selectSELECT 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 rowsDELETE FROM student WHERE ROWNUM <= 100

deletes at most 100 rows• ROWNUMs are assigned before the rows are orderedSELECT * 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 students student

ORDER BY NAME DESC)

WHERE ROWNUM <= 100

Oracle extensions• Hierarchical queries: for table that references itself• Special CONNECT BY clauseSELECT *

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

Hierarchical queries• Special LEVEL pseudo-column is available in hierarchical

queries:SELECT level, e.* FROM employers eCONNECT BY e.boss_id = PRIOR e.idSTART WITH e.id = 1;

Practice SQL statementscreate 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)