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.
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