Question

I am attempting to write a MySQL query that will return element whose details meet a this but not this criteria.

As an example consider a an inventory management system for office supplies. Supplies of all types would be contained within this database, and instead of creating a table with a myriad of columns each attribute of a product’s description I instead did this.

CREATE TABLE `inventory`(
id INTEGER AUTO_INCREMENT PRIMARY KEY,
element_name VARCHAR(255) NOT NULL
);

CREATE TABLE `inventory_detail`(
id INTEGER AUTO_INCREMENT PRIMARY KEY,
element_id INTEGER NOT NULL,
label VARCHAR(255) NOT NULL,
attribute_string VARCHAR(255) NULL,
attribute_integer INTEGER NULL
);

Using these tables each product can added as inventory and it’s attributes can be added as inventory detail. (ie. label = color, attribute_string = red or label = color, attribute_string != blue)
Don’t critique this method it is not what my question is about. Besides this method allows me to constantly change the number of details about a product that is stored while not having to change the database in any way.

My question is what should my query look like if I wanted to get a list of all green products that have a positive count. Basically I am looking for a query that returns element.id and the elments must be sorted by details that are excepted and ones that are not. For example:

SELECT element.id
FROM element
INNER JOIN element_detail ON element.id = element_detail.id
WHERE (element_detail.label = 'color'
AND element_detail.attribute_string = 'green')
AND (element_detail.label = 'count'
AND element_detail.attribute_integer <> 0);

####################################
#             INVENTORY            #
####################################
# id #           label             #
####################################
#  1 #   multi color pen pack      #
#  2 #   single pen pack (red)     #
#  3 #   single pen pack (green)   #
#  4 #   single pen pack (black)   #
#  5 #   single pen pack (blue)    #
#  6 #   single pen pack (purple)  #
####################################

########################################################
#                  INVENTORY_DETAILS                   #
########################################################
# id # element_id #    label   #    attribute_string   #
########################################################
#  1 #     1      #    color   #          red          #
#  2 #     1      #    color   #          blue         #
#  3 #     1      #    color   #          black        #
#  4 #     1      #    color   #          green        #
#  5 #     1      #    color   #          red          #
#  6 #     1      #    count   #          100          #
#  7 #     2      #    color   #          red          #
#  8 #     2      #    count   #          50           #
#  9 #     3      #    color   #          green        #
# 10 #     3      #    count   #          50           #
# 11 #     4      #    color   #          black        #
# 12 #     4      #    count   #          50           #
# 13 #     5      #    color   #          blue         #
# 14 #     5      #    count   #          0            #
# 15 #     5      #    color   #          purple       #
# 16 #     5      #    count   #          50           #
########################################################

A query that will return all products in the inventory and their count that have the color red but not green.

########################################
#              RESULT                  #
########################################
#       PRODUCT             #   COUNT  #
#  single pen pack (red)    #     50   #
#  single pen pack (black)  #     50   #
#  single pen pack (blue)   #     0    #
#  single pen pack (purple) #     50   #
########################################

While the mutli color pen pack has a red color pen it also has a green color pen, and should not be included in the result.

Answer 1

If you want all “green” products with a positive count then you will need to join twice on the detail table

SELECT element.id
FROM element
INNER JOIN element_detail e1 ON element.id = e1.id
INNER JOIN element_detail e2 ON element.id = e2.id
WHERE (
      e1.label = 'color'
  AND e1.attribute_string = 'green'
) AND (
      e2.label = 'count'
  AND e2.attribute_integer > 0
);