How can i improve query for efficiency in mysql - mysql

Question

Senario:
I'm working with two tables, (structure below)
expenses_tb person_expenses_tb
+----+-----------+ +----+------+-------------+-----------+--------+
| id | expenses | | id | year | expenses_id | person | amount |
+----+-----------+ +----+------+-------------+-----------+--------+
| 1 | Bus | | 1 | 2007 | 1 | Will | 20 |
| 2 | Food | | 2 | 2007 | 2 | Will | 200 |
| 3 | Clothes | | 3 | 2007 | 4 | Will | 1000 |
| 4 | Girlfriend| | 4 | 2007 | 5 | Will | 20 |
| 5 | Taxi | | 5 | 2008 | 3 | Will | 500 |
+----+-----------+ | 6 | 2008 | 5 | Will | 100 |
| 7 | 2008 | 2 | Holly | 200 |
| 8 | 2007 | 5 | Holly | 850 |
| .. | 2013 | ... | .... | ... |
I have tried two different queries.
SELECT person, expenses,
CASE expense_id WHEN 1 THEN amount END AS 'bus',
CASE expense_id WHEN 2 THEN amount END AS 'food',
CASE expense_id WHEN 3 THEN amount END AS 'clothes',
CASE expense_id WHEN 4 THEN amount END AS girlfriend',
CASE expense_id WHEN 5 THEN amount END AS 'taxi'
FROM person_expenses_tb p
JOIN expenses e ON e.id=p.expenses_id
WHERE p.year = 2008
GROUP BY p.person
The query above runs fast but does not produce the desired output.
Second query i tried was
SELECT person, expenses,
(SELECT amount FROM person_expenses_tb p_bus WHERE expense_id = 1 AND p_bus.person = p.person AND year=2008) AS 'bus',
(SELECT amount FROM person_expenses_tb p_bus WHERE expense_id = 2 AND p_bus.person = p.person AND year=2008) AS 'food',
(SELECT amount FROM person_expenses_tb p_bus WHERE expense_id = 3 AND p_bus.person = p.person AND year=2008) AS 'clothes',
(SELECT amount FROM person_expenses_tb p_bus WHERE expense_id = 4 AND p_bus.person = p.person AND year=2008) AS girlfriend',
(SELECT amount FROM person_expenses_tb p_bus WHERE expense_id = 5 AND p_bus.person = p.person AND year=2008) AS 'taxi'
FROM person_expenses_tb p
JOIN expenses e ON e.id=p.expenses_id
WHERE p.year = 2008
GROUP BY p.person
this query produced the right result but its extremely slow when the [person_expenses_tb] has over 2000 records.
the desired result for 2007: I have a i pass the requested year to the query.
+--------+------+-----+------+---------+------------+------+
| person | Year | Bus | Food | Clothes | Girlfriend | Taxi |
+--------+------+-----+------+---------+------------+------+
| Will | 2007 | 20 | 20 | 0 | 1000 | 20 |
| Holly | 2007 | 0 | 0 | 0 | 0 | 850 |
| ... | ... | ... | ... | ... | ... | ... |
i would love help on how to improve the query for it to run faster and if there is another way of getting the desired output i would appreciate the help.
Thank you.

Answer 1

The first query is the right approach (in general). You just need aggregation functions:
SELECT person,
sum(CASE p.expenses_id WHEN 1 THEN amount END) AS bus,
sum(CASE p.expenses_id WHEN 2 THEN amount END) AS food,
sum(CASE p.expenses_id WHEN 3 THEN amount END) AS clothes,
sum(CASE p.expenses_id WHEN 4 THEN amount END) AS girlfriend,
sum(CASE p.expenses_id WHEN 5 THEN amount END) AS taxi
FROM person_expenses_tb p
WHERE p.year = 2008
GROUP BY p.person;
Your version doesn't work because of a "feature" of MySQL. p.expenses_id is not in the group by clause. In that case, MySQL chooses an arbitrary value. Hence, only one of the columns will be populated. The aggregation function fixes this problem.
Because you are using expense_id for pivoting, you don't need the join to the reference table. I also removed the single quotes from the names of the columns. Although MySQL allows this, it is bad practice. Just use single quotes for string constants. If you need to escape a name, use back quotes or double quotes.
(I also changed expense_id to expenses_id to match the naming in your sample data.)