MySQL Combine row values with comma and more

问题

I have been banging my head over this for a while. Mainly it's not about getting a solution BUT how it works. It would be great if you can give explanation on how the solution is implemented.

This is just sample data, I am working with 5 tables with inline SELECT statements. Thank you SQL Gurus.

Sample Tables:

CREATE TABLE source
(
  product varchar(20),
  amount decimal
)

CREATE TABLE cat
(
  category varchar(20),
  product varchar(20)
)

Insert Sample Data:

INSERT INTO source (product, amount) VALUES
('product1', 1.00),
('product2', 2.00),
('product3', 1.50),
('product4', 5.25)

INSERT INTO cat (category, product) VALUES
('Shirts', 'product1'),
('Green Shirt', 'product1'),
('Gadget', 'product1'),
('Food', 'product2'),
('Games', 'product4')

My Trial Query:

DECLARE @categories varchar(200)
SET @categories = NULL

SELECT 
@categories = COALESCE(@categories + ',','') + cat.category
FROM cat

SELECT
DISTINCT source.product,
@categories
FROM cat
JOIN source on source.product = cat.product

OUTPUT

PRODUCT     COLUMN_1
product1    Shirts,Green Shirt,Gadget,Food,Games
product2    Shirts,Green Shirt,Gadget,Food,Games
product4    Shirts,Green Shirt,Gadget,Food,Games

DESIRED OUTPUT:

PRODUCT     COLUMN_1
product1    Shirts, Green Shirt, Gadget
product2    Gadget, Food
product4    Games

回答1:

I think this is what you're looking for using GROUP_CONCAT:

SELECT
  source.product,
  GROUP_CONCAT(cat.category) cats
FROM cat
  JOIN source on source.product = cat.product
GROUP BY source.product

SQL Fiddle Demo

If you're trying to get a ', ' in between each category, use SEPARATOR with GROUP_CONCAT. For example:

GROUP_CONCAT(cat.category separator ', ')

来源：https://stackoverflow.com/questions/16720129/mysql-combine-row-values-with-comma-and-more

标签

mysql

comma