Conditional IF function

In the previous lesson, we looked at the CASE statement for implementing conditional logic in SQL. However, this is not the only mechanism by which it is possible to implement logic branching in a query. It's time to turn our attention to the IF function.

IF Syntax

MySQL 8.4

IF(condition, value_if_true, value_if_false)

condition: The logical expression being checked, for example price > 100.
value_if_true: The value IF returns if the condition is true.
value_if_false: The value IF returns if the condition is false or NULL.

Additional conditional logic functions

In the previous lesson, we studied the CASE statement for implementing conditional logic in SQL. PostgreSQL provides additional functions that simplify working with conditional logic in special cases. These functions are especially useful when working with NULL values and creating more readable code.

Functions for conditional logic

In addition to the universal CASE operator, PostgreSQL provides:

COALESCE function - for working with NULL values
NULLIF function - for special cases with NULL

These functions are standard SQL functions and make code more readable in certain situations.

Examples

Simple comparison of two numbers. Since 10 is not greater than 20, the function will return 'FALSE'.
```
MySQL 8.4
SELECT IF(10>20, "TRUE", "FALSE");
```
IF(10>20, "TRUE", "FALSE")
FALSE

Simple example of conditional logic using the CASE operator from the previous lesson:

MySQL 8.4
SELECT CASE WHEN 10 > 20 THEN 'TRUE' ELSE 'FALSE' END;

case
FALSE

Example of use with a real database. Based on the price, it is necessary to determine whether the housing belongs to one of two classes: "Comfort class" and "Economy class". If the price is greater than or equal to 150, then this housing belongs to "Comfort class".

MySQL 8.4
SELECT id, price,
    IF(price >= 150, "Comfort class", "Economy class") AS category
    FROM Rooms

id	price	category
1	149	Economy class
2	225	Comfort class
3	150	Comfort class
4	89	Economy class
5	80	Economy class
6	200	Comfort class
7	60	Economy class
8	79	Economy class
9	79	Economy class
10	150	Comfort class
11	135	Economy class
12	85	Economy class
13	89	Economy class
14	85	Economy class
15	120	Economy class
16	140	Economy class
17	215	Comfort class
18	140	Economy class
19	99	Economy class
20	190	Comfort class
21	299	Comfort class
22	130	Economy class
23	80	Economy class
24	110	Economy class
25	120	Economy class
26	60	Economy class
27	80	Economy class
28	150	Comfort class
29	44	Economy class
30	180	Comfort class
31	50	Economy class
32	52	Economy class
33	55	Economy class
34	50	Economy class
35	70	Economy class
36	89	Economy class
37	35	Economy class
38	85	Economy class
39	150	Comfort class
40	40	Economy class
41	68	Economy class
42	120	Economy class
43	120	Economy class
44	135	Economy class
45	150	Comfort class
46	150	Comfort class
47	130	Economy class
48	110	Economy class
49	115	Economy class
50	80	Economy class

Example with real data. The CASE operator helps categorize housing by price:

MySQL 8.4
SELECT id, price,
    CASE WHEN price >= 150 THEN 'Comfort class' ELSE 'Economy class' END AS category
    FROM Rooms

id	price	category
1	149	Economy class
2	225	Comfort class
3	150	Comfort class
4	89	Economy class
5	80	Economy class
6	200	Comfort class
7	60	Economy class
8	79	Economy class
9	79	Economy class
10	150	Comfort class
11	135	Economy class
12	85	Economy class
13	89	Economy class
14	85	Economy class
15	120	Economy class
16	140	Economy class
17	215	Comfort class
18	140	Economy class
19	99	Economy class
20	190	Comfort class
21	299	Comfort class
22	130	Economy class
23	80	Economy class
24	110	Economy class
25	120	Economy class
26	60	Economy class
27	80	Economy class
28	150	Comfort class
29	44	Economy class
30	180	Comfort class
31	50	Economy class
32	52	Economy class
33	55	Economy class
34	50	Economy class
35	70	Economy class
36	89	Economy class
37	35	Economy class
38	85	Economy class
39	150	Comfort class
40	40	Economy class
41	68	Economy class
42	120	Economy class
43	120	Economy class
44	135	Economy class
45	150	Comfort class
46	150	Comfort class
47	130	Economy class
48	110	Economy class
49	115	Economy class
50	80	Economy class

IF functions can also be nested within each other, emulating the CASE operator.

MySQL 8.4
SELECT id, price,
    IF(price >= 200, "Business Class",
        IF(price >= 150,
            "Comfort class", "Economy class")) AS category
    FROM Rooms

id	price	category
1	149	Economy class
2	225	Business Class
3	150	Comfort class
4	89	Economy class
5	80	Economy class
6	200	Business Class
7	60	Economy class
8	79	Economy class
9	79	Economy class
10	150	Comfort class
11	135	Economy class
12	85	Economy class
13	89	Economy class
14	85	Economy class
15	120	Economy class
16	140	Economy class
17	215	Business Class
18	140	Economy class
19	99	Economy class
20	190	Comfort class
21	299	Business Class
22	130	Economy class
23	80	Economy class
24	110	Economy class
25	120	Economy class
26	60	Economy class
27	80	Economy class
28	150	Comfort class
29	44	Economy class
30	180	Comfort class
31	50	Economy class
32	52	Economy class
33	55	Economy class
34	50	Economy class
35	70	Economy class
36	89	Economy class
37	35	Economy class
38	85	Economy class
39	150	Comfort class
40	40	Economy class
41	68	Economy class
42	120	Economy class
43	120	Economy class
44	135	Economy class
45	150	Comfort class
46	150	Comfort class
47	130	Economy class
48	110	Economy class
49	115	Economy class
50	80	Economy class

CASE is perfect for multiple conditions:

MySQL 8.4
SELECT id, price,
    CASE
        WHEN price >= 200 THEN 'Business Class'
        WHEN price >= 150 THEN 'Comfort class'
        ELSE 'Economy class'
    END AS category
    FROM Rooms

id	price	category
1	149	Economy class
2	225	Business Class
3	150	Comfort class
4	89	Economy class
5	80	Economy class
6	200	Business Class
7	60	Economy class
8	79	Economy class
9	79	Economy class
10	150	Comfort class
11	135	Economy class
12	85	Economy class
13	89	Economy class
14	85	Economy class
15	120	Economy class
16	140	Economy class
17	215	Business Class
18	140	Economy class
19	99	Economy class
20	190	Comfort class
21	299	Business Class
22	130	Economy class
23	80	Economy class
24	110	Economy class
25	120	Economy class
26	60	Economy class
27	80	Economy class
28	150	Comfort class
29	44	Economy class
30	180	Comfort class
31	50	Economy class
32	52	Economy class
33	55	Economy class
34	50	Economy class
35	70	Economy class
36	89	Economy class
37	35	Economy class
38	85	Economy class
39	150	Comfort class
40	40	Economy class
41	68	Economy class
42	120	Economy class
43	120	Economy class
44	135	Economy class
45	150	Comfort class
46	150	Comfort class
47	130	Economy class
48	110	Economy class
49	115	Economy class
50	80	Economy class

However, for special cases, PostgreSQL provides more specialized functions.

IFNULL and NULLIF functions

In addition to the IF function, MySQL also has simpler, but less universal functions IFNULL and NULLIF, aimed at processing NULL values.

IFNULL syntax

MySQL 8.4

IFNULL(value, alternative_value)

value: The value being checked, for example a column that may contain NULL. If it is not NULL, the function returns it.
alternative_value: The value the function returns if value is NULL.

COALESCE function

The COALESCE function is an elegant solution for working with NULL values. It returns the first non-NULL value from the list of arguments.

Syntax

MySQL 8.4

COALESCE(value [, ...])

[ ]optional part
...can be repeated

value: Values separated by commas, for example table columns. They are checked from left to right, and you can put a fallback value last.

This is much more convenient than writing long CASE expressions to handle NULL.

Comparison of approaches

Using CASE:

MySQL 8.4
CASE
    WHEN value1 IS NOT NULL THEN value1
    WHEN value2 IS NOT NULL THEN value2
    ELSE value3
END

Using COALESCE (much simpler):

MySQL 8.4
COALESCE(value1, value2, value3)

Examples with the IFNULL function

Examples with the COALESCE function

If the first argument is not equal to NULL, then it will be returned.

MySQL 8.4
SELECT IFNULL("SQL Academy", "Alternative SQL Academy") AS sql_trainer;

sql_trainer
SQL Academy

If the first argument is not equal to NULL, then it will be returned.

MySQL 8.4
SELECT COALESCE('SQL Academy', 'Alternative SQL Academy') AS sql_trainer;

sql_trainer
SQL Academy

If the first argument is equal to NULL, then the value passed by the second argument will be returned.

MySQL 8.4
SELECT IFNULL(NULL, "Alternative SQL Academy") AS sql_trainer;

sql_trainer
Alternative SQL Academy

If the first argument is equal to NULL, then the next non-NULL value will be returned.

MySQL 8.4
SELECT COALESCE(NULL, 'Alternative SQL Academy') AS sql_trainer;

sql_trainer
Alternative SQL Academy

COALESCE can accept multiple arguments, making the code very readable:

MySQL 8.4
SELECT COALESCE(NULL, NULL, 'SQL Academy', 'Backup option') AS sql_trainer;

sql_trainer
SQL Academy

NULLIF function

The NULLIF function is useful when you need to replace a specific value with NULL. This can be helpful for filtering or processing "empty" values.

NULLIF syntax

MySQL 8.4

NULLIF(value_1, value_2)

value_1: The value the function returns if it is not equal to value_2.
value_2: The value to compare with. If value_1 equals it, the function returns NULL.

Examples with the NULLIF function

If the value of the first argument is equal to the value of the second argument, then NULL is returned.

MySQL 8.4
SELECT NULLIF("SQL Academy", "SQL Academy") AS sql_trainer;

sql_trainer
<NULL>

If the value of the first argument is equal to the value of the second argument, then NULL is returned.

MySQL 8.4
SELECT NULLIF('SQL Academy', 'SQL Academy') AS sql_trainer;

sql_trainer
<NULL>

If the values of the first and second arguments are different, then the value of the first argument is returned.

MySQL 8.4
SELECT NULLIF("SQL Academy", "Alternative SQL Academy") AS sql_trainer;

sql_trainer
SQL Academy

If the values of the first and second arguments are different, then the value of the first argument is returned.

MySQL 8.4
SELECT NULLIF('SQL Academy', 'Alternative SQL Academy') AS sql_trainer;

sql_trainer
SQL Academy

When to use each function:

CASE: When you need complex conditional logic with multiple conditions
COALESCE: When you need to replace NULL values with default values
NULLIF: When you need to turn specific values into NULL

These functions make code more readable and are part of the SQL standard.