How to select sql with if else use column - mysql

Question

I'm beginner for mysql and this my graduation project please help me.
//AMOUNT_PEOPLE is variable in nodejs
//place_id is variable in nodejs recieve from front end.
SELECT
(IF AMOUNT_PEOPLE >= 10
RETURN COLUMN package_tb.price_group*(AMOUNT_PEOPLE-1)
ELSE IF AMOUNT_PEOPLE >= 6
RETURN COLUMN package_tb.price_group*(AMOUNT_PEOPLE-1) - (SELECT option_tb.price_group FROM option_tb WHERE obtion_tb.place_id = place_id)
ElSE
RETURN COLUMN price_normal*AMOUNT_PEOPLE
END IF) AS price,name,detail
FROM package_tb WHERE package_tb.place_id = place_id
This ticket booking program
Logic is
Check number of tourist
if tourist >= 10 must use group_price and free 1 person include food for free person option
but if tourist >= 6 must use group_price
and free 1 person but not include food for free person option
finally tourist 0-5 must use normal_price
Such customer tell me "I want ticket for 10 tourist" the system will check as above explain.
package_tb
-package_id
-place_id
-name
-detail
-price_group
-price_normal
option_tb
-option_id
-place_id
-name
-price_group
-price_normal
place_tb
-place_id
-name
If tourist use price group option have to use price group only
But tourist use price normal option have to use price normal only.
Sorry for my bad english.

Answer 1

There exists a CASE function.
Which is a standard SQL function for switch logic.
Based on the updated question I'm assuming that there can be multiple options per place.
So with node.js variables:
SET #place_id = ${PLACE_ID};
SET #amount_people = ${AMOUNT_PEOPLE};
SELECT
CASE
WHEN #amount_people >= 10
THEN (p.price_group * (#amount_people - 1))
WHEN #amount_people >= 6
THEN (p.price_group * (#amount_people - 1)) - SUM(o.price_group)
ELSE p.price_normal
END AS price,
p.name,
p.detail
FROM package_tb p
LEFT JOIN option_tb o ON o.place_id = p.place_id
WHERE p.place_id = #place_id
GROUP BY p.package_id, p.place_id, p.price_normal, p.price_group, p.name, p.detail;
A test on rextester here

Answer 2

-- From which table you want use price_normal? tb or o ???
SELECT
CASE WHEN p.AMOUNT_PEOPLE >= 4
THEN p.price_group
WHEN p.AMOUNT_PEOPLE >= 3
THEN p.price_group - o.price_food
ELSE tb.price_normal
END AS price,
p.name, p.detail
FROM package_tb p JOIN option_tb o ON o.package_id = p.package_id;

Answer 3

There are two main control-flow functions in MySQL IF() and CASE
Since you are not comparing the value of AMOUNT_PEOPLE directly, CASE is a bit of overkill, which can be simplified slightly by using IF.
The syntax for IF is IF(<expr>, <true_result>, <false_result>). This allows you to perform else if by chaining another IF() as the false_result
IF(<expr>, ..., IF(<expr>, ..., ...))
Instead of using else if, you only need to remove the option_tb.price_group when AMOUNT_PEOPLE is fewer than 10 to get your desired pricing.
/* groups with 6 or more use group price and one person free */
IF(AMOUNT_PEOPLE >= 6,
/* groups with fewer than 10 people remove option */
p.price_group*(AMOUNT_PEOPLE-1) - IF(AMOUNT_PEOPLE < 10,
o.price_group,
0
),
p.price_normal*AMOUNT_PEOPLE
) AS price
This reduces the amount of code slightly, to determine when to subtract a person.
Instead of using a nested sub-query, which would be executed for each row returned. If the option_tb.place_id is unique, a JOIN would be more preferable.
If option_tb.place_id is not unique, you would need to use a GROUP BY. One approach is to JOIN using a sub-query, to avoid false matching on the join table groupings.
To ensure results are not excluded when a row in the option_tb table fails to match a place_id, you would use a LEFT JOIN that returns NULL instead of excluding the row.
Then you can use COALESE(<column>, 0) to retrieve the value from the column or 0 if the column value is NULL.
In NodeJS you can use ${var} to inject a variable into a string.
For example:
var place_id = 1;
var query = 'SELECT ${place_id};';
console.log(query);
Results in
SELECT 1;
Putting it all together.
Example: db-fiddle
SELECT
IF(${AMOUNT_PEOPLE} >= 6,
p.price_group*(${AMOUNT_PEOPLE}-1) - IF(${AMOUNT_PEOPLE} < 10, COALESCE(o.price_group, 0), 0),
p.price_normal*${AMOUNT_PEOPLE}
) AS price,
p.name,
p.detail
FROM package_tb AS p
LEFT JOIN (
SELECT
place_id,
SUM(price_group) AS price_group
FROM option_tb
GROUP BY place_id
) AS o
ON o.place_id = p.place_id
WHERE p.place_id = ${place_id};