mysql query become slow when data reached upto 100000 - php

Question

I build a query a month ago on a website. It was working fine. But after a month I was informed that the website become very slow to load the page.
When I search for the problem, I found that my query is executing very slow to fetch the data from mysql database. Then I check for the database and found that the 4 tables which I was using by joins, have around 216850, 167634, 64000, 931 rows respectively.
I have already have indexed that tables. So, where I'm lacking. Please help guys.
[Edit]
Table1: user_alert
Records: 216850
DB Type: InnoDB
Indexes: id(primary)
Table2: orders
Records: 167634
DB Type: InnoDB
Indexes: id(primary), order_id, customer_id
Table3: user_registration
Records: 64000 around
DB Type: InnoDB
Indexes: id(primary), email_address
Table4: cities
Records: 931
DB Type: InnoDB
Indexes: id(primary)
Query:
SELECT uas.alert_id, uas.user_id, uas.status, ur.first_name, ur.last_name, ur.email_address, o.order_id,
CASE WHEN ct.city_name IS NULL THEN uas.city_name ELSE ct.city_name END AS city_name
FROM `user_alert` uas
LEFT JOIN orders o ON o.customer_id = uas.user_id
LEFT JOIN user_registration ur ON ur.id = uas.user_id
LEFT JOIN `cities` ct ON ct.city_id = uas.city_id
WHERE uas.status = '1'
GROUP BY uas.user_id
ORDER BY uas.create_date DESC

Answer 1

GROUP BY is used to aggregate values up. For example if you wanted the count of orders by a user you could use COUNT(o.order_id).....GROUP BY uas.user_id. There are multiple orders for each user, but the aggregate function is just counting them here. However if you just select o.order_id when you have a GROUP BY uas.user_id it doesn't know which of the possibly many order_id values to return for that user id.
In this case it possibly doesn't matter as it looks like the order table is the only one where there is multiple rows per use. If you want the latest one you could just use MAX(o.order_id) (assuming that the order_id is assigned is order). But if you wanted the order value it becomes more difficult.
SELECT uas.alert_id, uas.user_id, uas.status, ur.first_name, ur.last_name, ur.email_address, MAX(o.order_id) AS LatestOrderId,
IFNULL(ct.city_name, uas.city_name) AS city_name
FROM `user_alert` uas
LEFT JOIN orders o ON o.customer_id = uas.user_id
LEFT JOIN user_registration ur ON ur.id = uas.user_id
LEFT JOIN `cities` ct ON ct.city_id = uas.city_id
WHERE uas.status = '1'
GROUP BY uas.user_id
ORDER BY uas.create_date DESC
If you wanted the (say) value of the latest order then it becomes more difficult.
SELECT uas.alert_id, uas.user_id, uas.status, ur.first_name, ur.last_name, ur.email_address, Sub1.MaxOrderId AS LatestOrderId, o.order_value
IFNULL(ct.city_name, uas.city_name) AS city_name
FROM `user_alert` uas
LEFT JOIN (SELECT customer_id, MAX(order_id) AS MaxOrderId FROM orders GROUP BY customer_id) Sub1 ON Sub1.customer_id = uas.user_id
LEFT OUTER JOIN orders o ON o.customer_id = Sub1.user_id AND o.order_id = Sub1.MaxOrderId
LEFT JOIN user_registration ur ON ur.id = uas.user_id
LEFT JOIN `cities` ct ON ct.city_id = uas.city_id
WHERE uas.status = '1'
ORDER BY uas.create_date DESC
Or doing a bit of a fiddle based on GROUP_CONCAT
SELECT uas.alert_id, uas.user_id, uas.status, ur.first_name, ur.last_name, ur.email_address,
SUBSTRING_INDEX(GROUP_CONCAT(o.order_id ORDER BY o.order_id DESC), ',', 1) AS LatestOrderId,
SUBSTRING_INDEX(GROUP_CONCAT(o.order_value ORDER BY o.order_id DESC), ',', 1) AS LatestOrderValue,
IFNULL(ct.city_name, uas.city_name) AS city_name
FROM `user_alert` uas
LEFT OUTER JOIN orders o ON o.customer_id = uas.user_id AND o.order_id = Sub1.MaxOrderId
LEFT JOIN user_registration ur ON ur.id = uas.user_id
LEFT JOIN `cities` ct ON ct.city_id = uas.city_id
WHERE uas.status = '1'
GROUP BY uas.user_id
ORDER BY uas.create_date DESC