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Data Malipulation Language Helpers

WHERE Clause

SQL-DML-Statement
FROM table_name
WHERE predicate

  • sql SELECT * FROM Customers WHERE Country='Mexico';

  • sql SELECT * FROM Customers WHERE CustomerID=1;

Predicates

  • =, > , <, >=, <=, <> not equal to
  • IN
  • BETWEEN
  • LIKE
  • IS NULL

  • IS NOT NULL

  • Join predicates

    • AND
    • OR
  • Invert Predicate
    • NOT

Operators

  • = equal to
  • > greater than
  • < less than
  • >= greater than and equal to
  • <= less than and equal to
  • <> not equal to

BETWEEN

SELECT ename WHERE ename BETWEEN 'value1' AND 'value2'

SELECT salary from emp WHERE salary BETWEEN 5000 AND 10000

AND, OR and NOT

  • WHERE clause can be combined with AND, OR, and NOT operators

IS NULL IS NOT NULL

SELECT column_names
FROM table_name
WHERE column_name IS NULL;

SELECT column_names
FROM table_name
WHERE column_name IS NOT NULL;

IN

SELECT column_name(s)
FROM table_name
WHERE column_name IN (value1, value2, ...);
  • nested queries
SELECT column_name(s)
FROM table_name
WHERE column_name IN (SELECT STATEMENT);

LIKE

  • used in the WHERE clause
  • MS access use * instead of %, and ? instead of _
SELECT column1, column2, ...
FROM table_name
WHERE columnN LIKE pattern;

Wildcards

You may have started to notice the variability in the implementations of different database now

  • MySQL
    • % - zero, one, or multiple characters
    • _ - one single character
    • [] - any single character within the brackets
    • ^ - any character not in the brackets
    • - - represents any single character within the specified range
  • MS Access
    • * - zero or more characters
    • ? - single character
    • [] - single character within the brackets
    • ! - represents character not in the bracket
    • - - represent any single character within the specified range
    • # - single numeric character

ORDER BY Clause

  • https://en.wikipedia.org/wiki/Order_by

  • sql SELECT * FROM Customers ORDER BY Country;

  • sql SELECT * FROM Customers ORDER BY Country DESC;

  • sql SELECT * FROM Customers ORDER BY Country, CustomerName;

  • sql SELECT * FROM Customers ORDER BY Country ASC, CustomerName DESC;

GROUP BY

SELECT column_name(s)
FROM table_name
WHERE condition
GROUP BY column_name(s)
ORDER BY column_name(s);

SELECT COUNT(CustomerID), Country
FROM Customers
GROUP BY Country
ORDER BY COUNT(CustomerID) DESC;

Aggregate Functions

SELECT MIN(column_name)
FROM table_name
WHERE condition;

SELECT MAX(column_name)
FROM table_name
WHERE condition;

SELECT COUNT(column_name)
FROM table_name
WHERE condition;

SELECT AVG(column_name)
FROM table_name
WHERE condition;

SELECT SUM(column_name)
FROM table_name
WHERE condition;

HAVING

  • https://en.wikipedia.org/wiki/Having_(SQL)
  • having clause was added because WHERE clause cannot be used with aggregate functions.
  • A HAVING clause in SQL specifies that an SQL SELECT statement must only return rows where aggregate values meet the specified conditions.
SELECT column_name(s)
FROM table_name
WHERE condition
GROUP BY column_name(s)
HAVING condition;
ORDER BY column_name(s);

SELECT DeptID, SUM(SaleAmount)
FROM Sales
WHERE SaleDate = '2000-01-01'
GROUP BY DeptID
HAVING SUM(SaleAmount) > 1000