Related
i have one table containing "Client" information, and another including "Tickets" information for each client.
int-------| varchar -------| varchar
client_id | client_name | client_tickets
----------+----------------+--------------
1 | Title one | 1,2
2 | Title two | 2,3
Simplified tickets table
int--------| varchar -------| varchar
ticket_id | ticket_name | ticket_price
-----------+-------------+--------------
1 | ticketone | 30
2 | tickettwo | 40
3 | ticketthree | 50
4 | ticketfour | 60
5 | ticketfive | 70
With the above two tables, I want to produce a single table with a single query with all the pertinent information to generate a search grid
So as to give the following output :
client_id | client_name | client_tickets | ticket_names | ticket_prices
----------+----------------+----------------+-----------------------+--
1 | Title one | 1,2 | ticketone,tickettwo | 30,40
2 | Title two | 2,3 | tickettwo,ticketthree | 40,50
ticket_names,ticket_ids,client_name are varchar
I want to receive the final 5 columns with one request
for example :
SELECT s.*,
(SELECT GROUP_CONCAT(ticket_name SEPARATOR ',') FROM tickets_table WHERE ticket_id IN(s.client_tickets)) AS ticket_names,
(SELECT GROUP_CONCAT(ticket_price SEPARATOR ',') FROM tickets_table WHERE ticket_id IN(s.client_tickets)) AS ticket_prices
FROM client_table s where s.client_id=1
Which seems to have a problem
Do you have a better suggestion?
Please make your suggestions
Update :
To clean the result I want
The following code has two querys,
I want this code to be done with a query
$client_result = $conn->query("SELECT * FROM client_table where client_id=1");
while($client_row = $client_result->fetch_assoc()) {
$ticket_result = $conn->query("SELECT * FROM tickets_table where ticket_id IN ($client_row['client_tickets'])");
while($ticket_row = ticket_result->fetch_assoc()) {
echo $ticket_row['ticket_name']."<br>";
}
}
update 2
i use suggest #raxi , but my mariadb is 10.4.17-MariaDB and don't support JSON_ARRAYAGG , for resolve it according to the reference Creating an aggregate function
, Using SQL
DELIMITER //
DROP FUNCTION IF EXISTS JSON_ARRAYAGG//
CREATE AGGREGATE FUNCTION IF NOT EXISTS JSON_ARRAYAGG(next_value TEXT) RETURNS TEXT
BEGIN
DECLARE json TEXT DEFAULT '[""]';
DECLARE CONTINUE HANDLER FOR NOT FOUND RETURN json_remove(json, '$[0]');
LOOP
FETCH GROUP NEXT ROW;
SET json = json_array_append(json, '$', next_value);
END LOOP;
END //
DELIMITER ;
What you want a fairly straightforward SELECT query with some LEFT/INNER JOIN(s).
This website has some good examples/explanations which seem very close to your need: https://www.mysqltutorial.org/mysql-inner-join.aspx
I would give you a quick working example, but it is not really clear to me what datatype the relevant columns are. Both tables' _id-columns are likely some variant of INTEGER, are they also both primary keys (or otherwise atleast indexed ?), the client_name/ticket_name are likely VARCHAR/TEXT/STRING types, but how exactly is the remaining column stored? as json or array or ? (+details)
Also you tagged your post with PHP, are you just after the SQL query ? or looking for PHP code with the SQL inside it.
updated
Improved version of the schema
CREATE TABLE clients (
client_id SERIAL,
client_name VARCHAR(255) NOT NULL,
PRIMARY KEY (client_id)
);
CREATE TABLE tickets (
ticket_id SERIAL,
ticket_name VARCHAR(255) NOT NULL,
ticket_price DECIMAL(10,2) NOT NULL,
PRIMARY KEY (ticket_id)
);
-- A junction table to glue those 2 tables together (N to N relationship)
CREATE TABLE client_tickets (
client_id BIGINT UNSIGNED NOT NULL,
ticket_id BIGINT UNSIGNED NOT NULL,
PRIMARY KEY (client_id, ticket_id)
);
I have changed the datatypes.
client_name and ticket_name are still VARCHARS. I've flagged them as NOT NULL (eg: required fields), but you can remove that part if you don't like that.
client_id/ticket_id/ticket_price are also NOT NULL but changing that has negative side-effects.
ticket_price is now a DECIMAL field, which can store numbers such as 1299.50 or 50.00 The (10,2) bit means it covers every possible number up to 8 whole digits (dollars/euros/whatever), and 2 decimals (cents). so you can store anything from $ -99.999.999,99 to $ 99.999.999,99 .
in SQL always write numbers (like lets say 70k) in this notation: 70000.00 (eg: a dot, not a comma; and no thousandseperators).
client_id and ticket_id are both SERIALs now, which is shorthand for BIGINT UNSIGNED NOT NULL AUTO_INCREMENT UNIQUE and theyre both PRIMARY KEYs on top of that. That probably sounds complicated but they're still just ordinary INTEGERs with values like 4 or 12 etc.
The UNIQUE bit prevents you from having 2 clients with the same ID number, and the AUTO_INCREMENT means that when you add a new client, you dont have to specify an ID (though you are allowed to); you can just do:
INSERT INTO clients (client_name) values ('Fantastic Mr Fox');
and the client_id will automatically be set (incrementing over time). And the same goes for ticket_id in the other table.
.
I've replaced your original client_tickets column, into a separate junction table.
Records in there store the client_id of a client and the ticket_id that belongs to them.
A client can have multiple records in the junction table (one record for each ticket they own).
Likewise, a ticket can be mentioned on any number of rows.
It's possible for a certain client_id to not have any records in the junction table.
Likewise, it's possible for a certain ticket_id to not have any records in the junction table.
Identical records cannot exist in this table (enforced by PRIMARY KEY).
Testdata
Next, we can put some data in there to be able to test it:
-- Create some tickets
INSERT INTO tickets (ticket_id, ticket_name, ticket_price) values (1, 'ticketone', '30' );
INSERT INTO tickets (ticket_id, ticket_name, ticket_price) values (2, 'tickettwo', '40' );
INSERT INTO tickets (ticket_id, ticket_name, ticket_price) values (3, 'ticketthree', '50' );
INSERT INTO tickets (ticket_id, ticket_name, ticket_price) values (4, 'ticketfour', '60' );
INSERT INTO tickets (ticket_id, ticket_name, ticket_price) values (5, 'ticketfive', '70' );
INSERT INTO tickets (ticket_id, ticket_name, ticket_price) values (6, 'ticketsix', '4' );
INSERT INTO tickets (ticket_id, ticket_name, ticket_price) values (7, 'ticketseven', '9' );
INSERT INTO tickets (ticket_id, ticket_name, ticket_price) values (8, 'ticketeight', '500' );
-- Create some users, and link them to some of these tickets
INSERT INTO clients (client_id, client_name) values (1, 'John');
INSERT INTO client_tickets (client_id, ticket_id) values (1, 3);
INSERT INTO client_tickets (client_id, ticket_id) values (1, 7);
INSERT INTO client_tickets (client_id, ticket_id) values (1, 1);
INSERT INTO clients (client_id, client_name) values (2, 'Peter');
INSERT INTO client_tickets (client_id, ticket_id) values (2, 5);
INSERT INTO client_tickets (client_id, ticket_id) values (2, 2);
INSERT INTO client_tickets (client_id, ticket_id) values (2, 3);
INSERT INTO clients (client_id, client_name) values (3, 'Eddie');
INSERT INTO client_tickets (client_id, ticket_id) values (3, 8);
INSERT INTO clients (client_id, client_name) values (9, 'Fred');
-- Note: ticket #3 is owned by both client #1/#2;
-- Note: ticket #4 and #6 are unused;
-- Note: client #9 (Fred) has no tickets;
Queries
Get all the existing relationships (ticket-less clients are left out & owner-less tickets are left out)
SELECT clients.*
, tickets.*
FROM client_tickets AS ct
INNER JOIN clients ON ct.client_id = clients.client_id
INNER JOIN tickets ON ct.ticket_id = tickets.ticket_id
ORDER BY clients.client_id ASC
, tickets.ticket_id ASC ;
Get all the tickets that are still free (owner-less)
SELECT tickets.*
FROM tickets
WHERE tickets.ticket_id NOT IN (
SELECT ct.ticket_id
FROM client_tickets AS ct
)
ORDER BY tickets.ticket_id ASC ;
Get a list of ALL clients (even ticketless ones), and include how many tickets each has and the total price of their tickets.
SELECT clients.*
, COALESCE(COUNT(tickets.ticket_id), 0) AS amount_of_tickets
, COALESCE(SUM(tickets.ticket_price), 0.00) AS total_price
FROM clients
LEFT JOIN client_tickets AS ct ON ct.client_id = clients.client_id
LEFT JOIN tickets ON ct.ticket_id = tickets.ticket_id
GROUP BY clients.client_id
ORDER BY clients.client_id ASC ;
Put all the juicy info together (owner-less tickets are left out)
SELECT clients.*
, COALESCE(COUNT(sub.ticket_id), 0) AS amount_of_tickets
, COALESCE(SUM(sub.ticket_price), 0.00) AS total_price
, JSON_ARRAYAGG(sub.js_tickets_row) AS js_tickets_rows
FROM clients
LEFT JOIN client_tickets AS ct ON ct.client_id = clients.client_id
LEFT JOIN (
SELECT tickets.*
, JSON_OBJECT( 'ticket_id', tickets.ticket_id
, 'ticket_name', tickets.ticket_name
, 'ticket_price', tickets.ticket_price
) AS js_tickets_row
FROM tickets
) AS sub ON ct.ticket_id = sub.ticket_id
GROUP BY clients.client_id
ORDER BY clients.client_id ASC ;
-- sidenote: output column `js_tickets_rows` (a json array) may contain NULL values
An list of all tickets with some aggregate data
SELECT tickets.*
, IF(COALESCE(COUNT(clients.client_id), 0) > 0
, TRUE, FALSE) AS active
, COALESCE( COUNT(clients.client_id), 0) AS amount_of_clients
, IF(COALESCE( COUNT(clients.client_id), 0) > 0
, GROUP_CONCAT(clients.client_name SEPARATOR ', ')
, NULL) AS client_names
FROM tickets
LEFT JOIN client_tickets AS ct ON ct.ticket_id = tickets.ticket_id
LEFT JOIN clients ON ct.client_id = clients.client_id
GROUP BY tickets.ticket_id
ORDER BY tickets.ticket_id ASC
, clients.client_id ASC ;
I have 5 tables related to sales. Three of them are like this:
Product_table_image
They are called product_a, product_b, product_c
The other tables are the time_id table which contains the reference for the date_id and the customers' table, which contains the details of customers.
time_id table
and
customer_table
The 3 tables refer to the sales of different products, but the products are not important in this context, because what I need is to sum up all the values per month per customer. There are cases when one or more customers might not have made a purchase of a certain product, which means not all customers ids will be in all products tables and that's what I can't figure out how to solve. It seems that my code is only able to fetch and sum when the clients have made purchases in all 3 tables.
So this is what I was able to come up with:
SELECT C.customer_id, ROUND((A.pa + B.pa + C.pc)* 1, 2) AS total,C.month_id
FROM (SELECT customer.customer_id, SUM(product_a.amount) AS pa , time_id.month_id FROM customer
INNER JOIN product_a on customer.customer_id = product_a.customer_id
INNER JOIN time_id on product_a.date_id = time_id.date_id
GROUP BY customer.customer_id, time_id.month_id) AS A
CROSS JOIN
(SELECT customer.customer_id, SUM(product_a.amount) AS pb , time_id.month_id FROM customer
INNER JOIN product_b on customer.customer_id = product_b.customer_id
INNER JOIN time_id on product_b.date_id = time_id.date_id
GROUP BY customer.customer_id, time_id.month_id) AS B
CROSS JOIN
(SELECT customer.customer_id, SUM(product_a.amount) AS pc , time_id.month_id FROM customer
INNER JOIN product_c on customer.customer_id = product_c.customer_id
INNER JOIN time_id on product_c.date_id = time_id.date_id
GROUP BY customer.customer_id, time_id.month_id) AS C
GROUP BY C.month_id, C.customer_id
ORDER BY C.month_id;
I've been stuck in it for a while, so any help is appreciated!
I have setup the tables and some sample data to make it more real.
create table month(id int, name varchar(20), primary key (id));
insert into month (id, name) values (1, 'January'),(2,'February'),(3,'March'),(4,'April');
create table year(id int, name varchar(4), primary key (id));
insert into year(id, name) values (2019, '2019'),(2020,'2020'),(2021,'2021');
create table time (id int, month_id int, year_id int, primary key (id));
alter table time add constraint fk_month FOREIGN KEY (month_id) REFERENCES month (id);
alter table time add constraint fk_year FOREIGN KEY (year_id) REFERENCES year (id);
insert into time (id, year_id, month_id) values
(1, 2019, 1),(2, 2019, 2),(3,2019,3),(4,2019,4),
(5, 2020, 1),(6, 2020, 2),(7,2020,3),(8,2020,4),
(9, 2021, 1),(10, 2021, 2),(11,2021,3),(12,2021,4);
create table customers (id int, name varchar(100), city varchar(100), country varchar(100), primary key (id));
insert into customers (id, name, city, country) values
(1, 'Google', 'San Francisco', 'US'),
(2, 'Ambev', 'São Paulo', 'BR'),
(3, 'Merck', 'Darmstadt', 'GE');
create table sales_of_product_a (id int, customer_id int, date_id int, amount decimal(10,2), primary key (id));
alter table sales_of_product_a add constraint fk_pa_time FOREIGN KEY (date_id) REFERENCES time (id);
-- only customer 1 - Google and 3 - Merck purchased product A
insert into sales_of_product_a (id, customer_id, date_id, amount) values
(1, 1, 1, 100.10),(2,1,2,200.20),(3,1,3,300.30),(4,1,4,400.40),
(5, 1, 5, 500.50),(6,1,6,600.60),(7,1,7,700.70),(8,1,8,800.80),
(9, 3, 1, 130.10),(10,3,2,230.20),(11,3,3,330.30),(12,3,4,430.40),
(13, 3, 5, 530.50),(14,3,6,630.60),(15,3,7,730.70),(16,3,8,830.80);
create table sales_of_product_b (id int, customer_id int, date_id int, amount decimal(10,2), primary key (id));
alter table sales_of_product_b add constraint fk_pb_time FOREIGN KEY (date_id) REFERENCES time (id);
-- only customer 1 - Google purchased product B
insert into sales_of_product_b (id, customer_id, date_id, amount) values
(1, 1, 1, 100.10),(2,1,2,200.20),(3,1,3,300.30),(4,1,4,400.40),
(5, 1, 5, 500.50),(6,1,6,600.60),(7,1,7,700.70),(8,1,8,800.80),
(9, 1, 9, 900.90),(10,1,10,1000.01),(11,1,11,1100.11),(12,1,12,1200.12);
create table sales_of_product_c (id int, customer_id int, date_id int, amount decimal(10,2), primary key (id));
alter table sales_of_product_c add constraint fk_pc_time FOREIGN KEY (date_id) REFERENCES time (id);
-- only customer 3 - Merck purchased product C
insert into sales_of_product_c (id, customer_id, date_id, amount) values
(1, 3, 1, 130.10),(2,3,2,230.20),(3,3,3,330.30),(4,3,4,430.40),
(5, 3, 5, 530.50),(6,3,6,630.60),(7,3,7,730.70),(8,3,8,830.80),
(9, 3, 9, 930.90),(10,3,10,1030.01),(11,3,11,1130.11),(12,3,12,1230.12);
The SQL you might be looking for would be something like.
with all_sales as (
select pa.customer_id, tt.month_id, sum(pa.amount) as amount from sales_of_product_a pa inner join time tt on (pa.date_id = tt.id) where tt.year_id = 2019 group by pa.customer_id, tt.month_id
union all
select pb.customer_id, tt.month_id, sum(pb.amount) as amount from sales_of_product_b pb inner join time tt on (pb.date_id = tt.id) where tt.year_id = 2019 group by pb.customer_id, tt.month_id
union all
select pc.customer_id, tt.month_id, sum(pc.amount) as amount from sales_of_product_c pc inner join time tt on (pc.date_id = tt.id) where tt.year_id = 2019 group by pc.customer_id, tt.month_id
),
sales_per_customer_per_month as (
-- summary of all sales of all products per customer per month
select customer_id, month_id, sum(amount) as amount from all_sales group by customer_id, month_id
),
customers_month as (
select c.id, c.name, c.city, c.country, m.id as month_id, m.name as month_name from customers c inner join month m on true
)
select c.id,c.name,c.city,c.country,c.month_id, coalesce(s.amount,0) as amount, sum(coalesce(s.amount,0)) over (partition by c.id order by c.id,c.month_id) as total
from customers_month c
left join sales_per_customer_per_month s on (s.customer_id = c.id and s.month_id = c.month_id)
order by c.id,c.month_id;
The result of query above is following.
The concepts used are linked below.
Window Functions:
https://dev.mysql.com/doc/refman/8.0/en/window-functions.html
Common Table Expressions (CTE):
https://www.mysqltutorial.org/mysql-cte/
I am using MySQL 5.6
Note:
TableA is Active_Orders
TableB is Old_Orders
TableC is Move_Orders
The field "contract" is unique in all three tables.
Here is what I got so far
Database setup:
CREATE TABLE IF NOT EXISTS Active_Orders (
id INT AUTO_INCREMENT PRIMARY KEY,
name VARCHAR(255) NOT NULL,
contract VARCHAR(255) NOT NULL UNIQUE
) ENGINE=INNODB;
INSERT INTO Active_Orders(name, contract)
VALUES ('steve', '3454'),
('tom', '6756'),
('becky', '9809');
CREATE TABLE IF NOT EXISTS Old_Orders (
id INT AUTO_INCREMENT PRIMARY KEY,
name VARCHAR(255) NOT NULL,
contract VARCHAR(255) NOT NULL UNIQUE
) ENGINE=INNODB;
INSERT INTO Old_Orders(name,contract)
VALUES ('mark', '9896'),
('kelly', '0897'),
('paul', '1537'),
('will', '8254');
CREATE TABLE IF NOT EXISTS Move_Orders (
contract VARCHAR(255) NOT NULL UNIQUE
) ENGINE=INNODB;
INSERT INTO Move_Orders(contract)
VALUES ('0897'),
('1537);
The Code I am using is:
INSERT INTO Active_Orders (name, contract)
SELECT name, contract
FROM Old_Orders
WHERE Move_Orders.contract = Old_Orders.contract;
But I am getting
#1054 - Unknown column 'Move_Orders.contract' in 'where clause'
What I want the result to be is:
SELECT * FROM Active_Orders;
id name contract
1 steve 3454
2 tom 6756
3 becky 9809
4 kelly 0897
5 paul 1537
I understand that 'Move_Orders.contract' is not in the FROM clause so I am getting error, but I am not sure how to rewrite the statement to get the output I need.
INSERT INTO Active_Orders (name, contract)
SELECT Old_Orders.name, Old_Orders.contract
FROM Old_Orders
JOIN Move_Orders ON Move_Orders.contract = Old_Orders.contract;
or
INSERT INTO Active_Orders (name, contract)
SELECT Old_Orders.name, Old_Orders.contract
FROM Old_Orders, Move_Orders
WHERE Move_Orders.contract = Old_Orders.contract;
or
INSERT INTO Active_Orders (name, contract)
SELECT name, contract
FROM Old_Orders
WHERE EXISTS ( SELECT contract
FROM Move_Orders
WHERE Move_Orders.contract = Old_Orders.contract )
It's nearly a literal translation from English:
INSERT INTO Active_Orders (name, contract)
SELECT o.name, o.contract
FROM Old_Orders o
WHERE o.contract IN (SELECT contract FROM Move_Orders)
You could also do a INNER JOIN between old_orders and move_orders, which will cause moveorders to filter the list of oldorders down to just those also present in move_orders
INSERT INTO Active_Orders (name, contract)
SELECT o.name, o.contract
FROM
Old_Orders o
INNER JOIN Move_Orders m ON o.contract = m.contract
I was wondering why my code isn't working. I'm trying to retrieve all offers of a listing only.
CREATE DATABASE IF NOT EXISTS `assignment_db`;
USE `assignment_db`;
CREATE TABLE USER_LIST(
id INT(6) UNSIGNED AUTO_INCREMENT PRIMARY KEY,
userName VARCHAR(50) NOT NULL,
email varchar(100) NOT NULL,
registeredDate timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP
);
create table listing_list(
id INT(6) UNSIGNED AUTO_INCREMENT PRIMARY KEY,
itemName VARCHAR(50) NOT NULL,
itemDescription VARCHAR(254) NOT NULL,
price DECIMAL(4,2) NOT NULL,
fk_poster_id int references USER_LIST(id),
created_at timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP
);
create table offer_list(
id int(6) Unsigned auto_increment Primary key,
offer int,
fk_listing_id int references listing_list(id),
fk_offeror_id int references user_list(id),
created_at timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP
);
insert into user_list (userName, email) values ('John','johnnyboi#123.com');
insert into user_list (userName, email) values ('Tom','Tommyboi#123.com');
insert into listing_list (itemName,itemDescription, price) values ( 'Pen', 'A long delicate pen.',' 1.50 ');
insert into listing_list (itemName,itemDescription, price) values ( 'Pencil', 'A long delicate pencil.',' 0.50 ');
insert into offer_list (offer,fk_listing_id,fk_offeror_id) values ('200','2','3');
insert into offer_list (offer,fk_listing_id,fk_offeror_id) values ('200','1','1');
select * from listing_list
inner join offer_list on listing_list.fk_listing_id = offer_list.id;
Any ideas on how to resolve this?
If I've understood you correctly, you want something like this (see fiddle here). Note that I changed some of the numbers to make it easier to follow the flow of the query:
insert into user_list (id, userName, email) values (10, 'John','johnnyboi#123.com');
insert into user_list (id, userName, email) values (20, 'Tom','Tommyboi#123.com');
and
insert into listing_list (itemName,itemDescription, price, fk_poster_id)
values ( 'Pen', 'A long delicate pen.',' 1.50 ', 10);
insert into listing_list (itemName,itemDescription, price, fk_poster_id)
values ( 'Pencil', 'A long delicate pencil.',' 0.50 ', 20);
and
insert into offer_list (offer,fk_listing_id,fk_offeror_id)
values ('200','1', 10);
insert into offer_list (offer,fk_listing_id,fk_offeror_id)
values ('200','2', 20);
Then I ran the query:
SELECT
u.id,
u.username,
u.email,
l.id,
l.itemName,
l.itemDescription,l.price,
l.fk_poster_id,
l.created_at,
o.offer,
o.fk_listing_id
FROM user_list u
JOIN listing_list l ON u.id = l.fk_poster_id
JOIN offer_list o ON u.id = o.fk_offeror_id;
Result:
id username email id itemName itemDescription price fk_poster_id created_at offer fk_listing_id
10 John johnnyboi#123.com 1 Pen A long delicate pen. 1.50 10 2019-12-23 11:21:10 200 1
20 Tom Tommyboi#123.com 2 Pencil A long delicate pencil. 0.50 20 2019-12-23 11:21:10 200 2
Some of the result fields are redundant - I left them in so that you could see the reasoning. Can I recommend that in future a) you don't bother with backticks for MySQL anymore - they are no longer required and are a pain to type. Also, when dealing with numberic values - even though MySQL (weirdly) supports this, that you do not put numbers in single quotes - it makes the SQL more difficult to read using many SQL formatting tools.
select * from listing_list inner join offer_list on listing_list.id = offer_list.fk_listing_id
You have taken the wrong table names.
For more details, check https://www.w3schools.com/sql/sql_join_inner.asp
The basic syntax for inner join:
SELECT column_name(s)
FROM table1
INNER JOIN table2
ON table1.column_name = table2.column_name;
If you want to select the data only form 1 listing, you can do it
like:
SELECT
*
FROM
listing_list
INNER JOIN offer_list ON listing_list.id = offer_list.fk_listing_id
WHERE
listing_list.id = 1
I have two mysql greatest-n-per-group, greatest-by-date problems:
Considering one students table and one grades table, I want to have all students displayed with their most recent grade.
The schema script:
CREATE TABLE student (
id int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(255) NOT NULL,
PRIMARY KEY (id)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
INSERT INTO student VALUES(1, 'jim');
INSERT INTO student VALUES(2, 'mark ');
INSERT INTO student VALUES(3, 'john');
CREATE TABLE grades (
id int(11) NOT NULL AUTO_INCREMENT,
student_id int(11) NOT NULL,
grade int(11) NOT NULL,
`date` date DEFAULT NULL,
PRIMARY KEY (id)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
INSERT INTO grades VALUES(1, 1, 6, NULL);
INSERT INTO grades VALUES(2, 1, 8, NULL);
INSERT INTO grades VALUES(3, 1, 10, NULL);
INSERT INTO grades VALUES(4, 2, 9, '2016-05-10');
INSERT INTO grades VALUES(5, 2, 8, NULL);
INSERT INTO grades VALUES(6, 3, 6, '2016-05-26');
INSERT INTO grades VALUES(7, 3, 7, '2016-05-27');
A) I want to find out if this is a valid solution for getting the most recent record by a date field (date) from a secondary table (grades) grouped for each row in a main table (student).
My query is:
SELECT s.id, s.name, g.grade, g.date
FROM student AS s
LEFT JOIN (
SELECT student_id, grade, DATE
FROM grades AS gr
WHERE DATE = (
SELECT MAX(DATE)
FROM grades
WHERE student_id = gr.student_id
)
GROUP BY student_id
) AS g ON s.id = g.student_id
Sql Fiddle: http://sqlfiddle.com/#!9/a84171/2
This query displays the desired (almost) results. But I have doubts that this is the best approach because it looks ugly, so I am very curious about the alternatives.
B) The second problem is the reason for the (almost) above,
For the first row, name=Jim it finds no grade though we have grades for Jim.
So just in case the query above would be valid only for NOT NULL date fields.
The question would be:
How to get the most recent grade for all the students who have grades, including Jim even that his grades has no date specified (NULL). In this case the most recent grouping will be given by the latest row inserted (MAX(id)) or just random.
Doesn't work with replacing date = (SELECT... with date IN (SELECT ....
Any help would be much appreciated,
Thanks!
[UPDATE #1]:
For B) I found adding this to the sub-query, OR date IS NULL, produces the desired result:
SELECT s.id, s.name, g.grade, g.date
FROM student AS s
LEFT JOIN (
SELECT id, student_id, grade, DATE
FROM grades AS gr
WHERE DATE = (
SELECT MAX(DATE)
FROM grades
WHERE student_id = gr.student_id
) OR date IS NULL
GROUP BY student_id
) AS g ON s.id = g.student_id
[UPDATE #2]
Seems the previous update worked if the first grade has a date for a student. It doesn't if the first grade is null. I would have linked a fiddle but it seems sqlfiddle doesn't work now.
So this is what I came up until now that seems to solve the B) problem:
SELECT s.id, s.name, g.grade, g.date
FROM student AS s
LEFT JOIN (
SELECT id, student_id, grade, DATE
FROM grades AS gr
WHERE (
`date` = (
SELECT MAX(DATE)
FROM grades
WHERE student_id = gr.student_id
)
) OR (
(
SELECT MAX(DATE)
FROM grades
WHERE student_id = gr.student_id
) IS NULL AND
date IS NULL
)
) AS g ON s.id = g.student_id
GROUP BY student_id
I still would like to know if you guys know better alternatives to this ugly thing.
Thanks!
[UPDATE #3]
#Strawberry
The desired results would be:
id name grade date
1 jim 10 NULL
2 mark 9 2016-05-10
3 john 7 2016-05-27
each student with one corresponding grade
if a date exists for a grade, then get the most recent one.
The complexity of this problem stems from the logical impossibility of a grade without an associated date, so obviously the solution is to fix that.
But here's a workaround...
E.g.:
SELECT a.*
FROM grades a
JOIN
( SELECT student_id
, MAX(COALESCE(UNIX_TIMESTAMP(date),id)) date
FROM grades
GROUP
BY student_id
) b
ON b.student_id = a.student_id
AND b.date = COALESCE(UNIX_TIMESTAMP(a.date),id);
http://sqlfiddle.com/#!9/ecec43/4
SELECT s.id, s.name, g.grade, g.date
FROM student AS s
LEFT JOIN (
SELECT gr.student_id, gr.grade, gr.DATE
FROM grades AS gr
LEFT JOIN grades grm
ON grm.student_id = gr.student_id
AND grm.date>gr.date
WHERE grm.student_id IS NULL
AND gr.date IS NOT NULL
GROUP BY gr.student_id
) AS g
ON s.id = g.student_id;