Lọc bằng WHERE

Often we do not need all the rows of a table, but only those that satisfy a certain condition. The WHERE clause filters rows based on a boolean expression:

SELECT <colonne>
FROM   <tabella>
WHERE  <condizione>;

The engine evaluates the condition one row at a time and keeps only those for which it returns TRUE.

Comparison operators

The most common ones are:

• = equal, <> (or !=) not equal
• <, <=, >, >=
• BETWEEN a AND b — closed interval (a and b included)
• IN (v1, v2, …) — membership in a list
• LIKE 'patt%' — text matching with wildcard %

Text values are wrapped in single quotes: 'Milano'.

Combining conditions

You can combine multiple conditions with AND, OR and negate them with NOT. When mixing them, use parentheses for clarity:

WHERE department_id = 1
  AND (salary > 40000 OR hired_on < '2020-01-01');

Try it

Review exercise