Conditional operator WHERE

The situation when you want to make a selection according to a certain condition is very common. To do this, there is a WHERE parameter in the SELECT statement, followed by a condition for restricting rows. If the record satisfies this condition, it falls into the result, otherwise it is discarded.

General query structure with the WHERE operator

SELECT table_fields FROM list_of_tables 
WHERE the_conditions_of_the_limitations
[logical_operator the_other_conditions_of_the_limitations];

In the described query structure, optional parameters are indicated in square brackets.

The conditional operator uses comparison operators, special operators, and logical operators.

Comparison operators

Comparison operators are used to compare 2 expressions, their result can be TRUE (1), FALSE (0) and NULL.

Result of comparison with NULL is NULL. The exception is the equivalence operator.
=The equality operator
<=>The operator equivalence.
Similar to the equality operator, with only one exception: in contrast, the equivalence operator will return TRUE when comparing NULL <=> NULL
The inequality operator
<Less operator
<=Less than or equal to operator
>Greater operator
>=Greater than or equal to operator

Special operators

1. IS [NOT] NULL — allows you to find out if the checked value is NULL.

As an example, we derive all family members whose family status is not NULL:

SELECT * FROM FamilyMembers

2. [NOT] BETWEEN min AND max — allows you to find out if the checked column value is located between min and max.

Display all the data on purchases with a price from 100 to 500 from the Payments table:

SELECT * FROM Payments
WHERE unit_price BETWEEN 100 AND 500;

3. [NOT] IN — allows you to find out whether the checked value of a column is included in the list of defined values.

Print the names of family members whose status is "father" or "mother":

SELECT member_name FROM FamilyMembers
WHERE status IN ('father', 'mother');

4. [NOT] LIKE pattern [ESCAPE symbol] — allows you to find out if a string matches a specific pattern.

For example, output all people with the surname "Quincey":

SELECT name FROM FamilyMembers
WHERE name LIKE '% Quincey';

Wildcard symbols

The template is allowed to use two wildcard symbols:

  • the underscore symbol (_), which can be used instead of any single character in the checked value
  • the percent symbol (%) replaces the sequence of any characters (the number of characters in the sequence can be from 0 or more) in the checked value.
never%Matches any line starting with "never".
%ingMatches any line ending with «ing».
_ingMatches strings with a length of 4 characters, while the last 3 must be "ing". For example, the words "sing" and "wing".

ESCAPE symbol

The ESCAPE symbol is used to escape the wildcard characters. In case you need to find strings containing percentages (and percent is a reserved character), you can use ESCAPE symbol.

For example, you want to get identifiers for tasks whose progress is 3%:

SELECT job_id FROM Jobs
WHERE progress LIKE '3!%' ESCAPE '!';

If we had not screened the wildcard character, then everything that starts at 3 would fall into the selection.

Logical operators

Logical operators are required for string constraint condition relationships.

  • Оператор NOT — changes the value of the special operator to the opposite
  • Оператор OR — the general meaning of the expression is true if at least one of them is true
  • Оператор AND — the general meaning of an expression is true if they are both true
  • Оператор XOR — the general meaning of the expression is true if one and only one argument is true

We will derive all the flights that were completed on a Boeing aircraft, but at the same time, the flight was not from London:

WHERE plane = 'Boeing' AND NOT town_from = 'London';

Tasks for self-testing

The syntax of the SQL query
Sorting, ORDER BY operator