Mysql – Selecting with multiple ands

I personally would use a model similar to the following:

The product table would be pretty basic, your main product details:

create table product
(
  part_number int, (PK)
  name varchar(10),
  price int
);
insert into product values
(1, 'product1', 50),
(2, 'product2', 95.99);

Second the attribute table to store the each of the different attributes.

create table attribute
(
  attributeid int, (PK)
  attribute_name varchar(10),
  attribute_value varchar(50)
);
insert into attribute values
(1, 'color', 'red'),
(2, 'color', 'blue'),
(3, 'material', 'chrome'),
(4, 'material', 'plastic'),
(5, 'color', 'yellow'),
(6, 'size', 'x-large');

Finally create the product_attribute table as the JOIN table between each product and its attributes associated with it.

create table product_attribute
(
  part_number int, (FK)
  attributeid int  (FK) 
);
insert into product_attribute values
(1,  1),
(1,  3),
(2,  6),
(2,  2),
(2,  6);

Depending on how you want to use the data you are looking at two joins:

select *
from product p
left join product_attribute t
  on p.part_number = t.part_number
left join attribute a
  on t.attributeid = a.attributeid;

See SQL Fiddle with Demo. This returns data in the format:

PART_NUMBER | NAME       | PRICE | ATTRIBUTEID | ATTRIBUTE_NAME | ATTRIBUTE_VALUE
___________________________________________________________________________
1           | product1   | 50    | 1           | color          | red
1           | product1   | 50    | 3           | material       | chrome
2           | product2   | 96    | 6           | size           | x-large
2           | product2   | 96    | 2           | color          | blue
2           | product2   | 96    | 6           | size           | x-large

But if you want to return the data in a PIVOT format where you have one row with all of the attributes as columns, you can use CASE statements with an aggregate:

SELECT p.part_number,
  p.name,
  p.price,
  MAX(IF(a.ATTRIBUTE_NAME = 'color', a.ATTRIBUTE_VALUE, null)) as color,
  MAX(IF(a.ATTRIBUTE_NAME = 'material', a.ATTRIBUTE_VALUE, null)) as material,
  MAX(IF(a.ATTRIBUTE_NAME = 'size', a.ATTRIBUTE_VALUE, null)) as size
from product p
left join product_attribute t
  on p.part_number = t.part_number
left join attribute a
  on t.attributeid = a.attributeid
group by p.part_number, p.name, p.price;

See SQL Fiddle with Demo. Data is returned in the format:

PART_NUMBER | NAME       | PRICE | COLOR | MATERIAL | SIZE
_________________________________________________________________
1           | product1   | 50    | red   | chrome   | null
2           | product2   | 96    | blue  | null     | x-large

As you case see the data might be in a better format for you, but if you have an unknown number of attributes, it will easily become untenable due to hard-coding attribute names, so in MySQL you can use prepared statements to create dynamic pivots. Your code would be as follows (See SQL Fiddle With Demo):

SET @sql = NULL;
SELECT
  GROUP_CONCAT(DISTINCT
    CONCAT(
      'MAX(IF(a.attribute_name = ''',
      attribute_name,
      ''', a.attribute_value, NULL)) AS ',
      attribute_name
    )
  ) INTO @sql
FROM attribute;

SET @sql = CONCAT('SELECT p.part_number
                    , p.name
                    , ', @sql, ' 
                   from product p
                   left join product_attribute t
                     on p.part_number = t.part_number
                   left join attribute a
                     on t.attributeid = a.attributeid
                   GROUP BY p.part_number
                    , p.name');

PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;

This generates the same result as the second version with no need to hard-code anything. While there are many ways to model this I think this database design is the most flexible.

I first thought that you needed to GROUP BY the datetable in a derived table as well, to avoid a cross join but it seems that the datetable already has distinct dates so that is not the cause. The problem is the <= in the (more than one) joins. This creates a kind of cross join and wrong results.

So, the solution is to do the LEFT JOIN and the <= inside the derived tables and in the external level join with equality = and no GROUP BY:

Another issue comes from not using GROUP BY DATE(DateColumn). Use that - and then the ON conditions can be simplified as well.

I also changed the DATE(tx.created) <= DATE(dt.thedate) condition to tx.created < (dt.thedate + INTERVAL 1 DAY) so an index on (created, hours) can be used:

SELECT
DATE_FORMAT(dt.thedate, '%Y-%m-%d') AS the_date,
COALESCE(tt.sum_task, 0) AS sum_t,
COALESCE(tg.sum_group, 0) AS sum_g

FROM datetable AS dt

LEFT JOIN   (                                  -- this could be an INNER JOIN, no difference
                SELECT
                dt.thedate,
                SUM(tx.hours) as sum_task

                FROM datetable AS dt
                  LEFT JOIN tasks AS tx
                    ON tx.created < (dt.thedate + INTERVAL 1 DAY)

                 -- Some extra where clauses here   -- move them to the ON above

                GROUP BY dt.thedate
            )
            AS tt ON tt.thedate = dt.thedate

LEFT JOIN   (                                 -- this could be an INNER JOIN, no difference
                SELECT
                dt.thedate,
                SUM(gx.hours) as sum_group

                FROM datetable AS dt
                  LEFT JOIN groups AS gx
                    ON gx.created < (dt.thedate + INTERVAL 1 DAY)

                 -- Some extra where clauses here   -- move them to the ON above

                GROUP BY dt.thedate
            )
            AS tg ON tg.thedate = dt.thedate

ORDER BY dt.thedate ;

Tested at SQLFiddle