SQL learning

Five basic search conditions are summarized here:
1) Comparison test
2) Range test
3) Set membership test
4) Pattern matching test (Like)
     The pattern matching test checks to see whether the data value in a column matches a specified pattern
   % mathes any sequence of zero or more characters.
   _ matches any single character.
// Find products whose product IDs start with the four letters "A%BC"
      SELECT order_num ,product
       from orders
      WHERE product LIKE 'A$%BC' ESCAPE '$' ;
5) Null value test (IS NULL)
In SQL's  three-valued logic, a search condition can yield a TRUE,FALSE, or NULL result. Only rows where search codition yields a TRUE result are included in the query result.
Rules For Single-Table Query Processing
The from clause is applied first; the where clause is applied next ; the SELECT clause is applied next . Finally, the
ORDER BY clause is applied to sort the query results.