Databse 2. SQL basic

SQL basic

Database Reminder

SQL Queries

• SQL Queries are how we get information from a relational database
• Start with SELECT command
• E.g. SELECT name FROM Student;

Structure of a Query

• basic form of a query:
  SELECT column_name FROM table_name WHERE condition_is_true;
• SELECT * ----> returns complete rows

Examples

• E.g. SELECT* FROM Lecturer
  SELECT* FROM Student
• E.g. SELECT fname,sname,address FROM Student WHERE fname = 'Sally';
• E.g. SELECT staffID FROM Lecturer WHERE school = 'Computing Science';
• E.g. SELECT* FROM Student WHERE fname = 'Sally';

SQL Case Sensitivity

• SELECT* FROM Student
• SQL commands are not case sensitive
• Convention is :
  -- SQL commends are in capitals(e.g. SELECT, FROM)
  -- Table names start with a capital letter (e.g.Student)
  -- Column names are in lower case (e.g. name, gender)

SELECT DISTINCT

• get only distinct column values(delete the repeat results, or something dupicate)
• SELECT DISTINCT fname FROM People;

Operators

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IN and BETWEEN

• IN: list possible values:
  SELECT name FROM Student WHEREstudentID IN(1,3,7);
• BETWEEN: between an inclusive range
  SELECT name FROM Student WHERE studentID BETWEEN 1 AND 20;
• NOT IN and NOT BETWEEN: similar as above

IS NULL

• NULL values represent missing data, it's different from an empty string or 0
• can use IS NULL and IS NOT NULL in queries

AND and OR

• AND means all conditions must be true
• OR means just one of the conditions must be true
• e.g. SELECT studentID FROM Student WHERE fname='Sally' AND address='12 Hope Street';
• e.g. SELECT studentID FROM Student WHERE fname='Sally' OR fname= 'Lindsey'

ORDER BY

• order reuslts
• e.g. SELECT coulumn_name,column_name FROM talbe_name ORDER BY column_name ASC|DESC;
• ASC is defult

Functions

• functions can be used for calculating the data
• Syntax : SLECT function(column) FROM Table_name;
• e.g. SELECT AVG(salary) FROM Lecturer;
• useful aggregate functions:

• AVG()
• MAX()
• MIN()
• SUM()
• COUNT()

COUNT()

• SELECT COUNT(*) FROM Employee;
• SELECT COUNT(salary) FROM Employee;
• SELECT COUNT(DISTINCT salary) FROM Employee;
• SELECT COUNT(staffID) FROM Lecturer WHERE school='Philosophy';

GROUP BY

• returens values for distinct groups
• e.g. SELECT COUNT(staffID), school FROM Lecturer GROUP BY school;----------> returns the number of lecturers in each school, one row for each distinct school

LIMIT

• want a specific number of row back
• e.g. SELECT fname, sname FROM Student WHERE sname< 'Black' LIMIT 10;

SQL Resources

• postgresql tutorial