Question

Update
Here is the ddl of the two tables that the CASE statement uses

CREATE TABLE product_option (
merchant_id                 smallint unsigned NOT NULL,
product_option_id           smallint unsigned NOT NULL AUTO_INCREMENT,
product_option_name         varchar(255) NOT NULL DEFAULT '',
product_id                  int unsigned NOT NULL DEFAULT 0,    /* Option may be associated with a product  */
package_id                  int unsigned NOT NULL DEFAULT 0,    /* or all products with this package    */
PRIMARY KEY pk_product_option (merchant_id,product_option_id),

) TYPE=MyISAM PACK_KEYS=1 COMMENT='Options';

CREATE TABLE package_2_product (
merchant_id     smallint unsigned NOT NULL,
package_id      int unsigned NOT NULL,
product_id      int unsigned NOT NULL,

PRIMARY KEY pk_package_2_product (merchant_id,package_id,product_id)

) TYPE=MyISAM PACK_KEYS=1 COMMENT='Link product to package';

I have a query…

SELECT SQL_CALC_FOUND_ROWS 
  p.*,
  CASE p.in_stock_msg  WHEN '' THEN 'In stock' ELSE p.in_stock_msg  END AS in_stock_msg, 
  CASE p.out_stock_msg WHEN '' THEN ''         ELSE p.out_stock_msg END AS out_stock_msg,
  CASE WHEN EXISTS (
    SELECT product_option_id 
      FROM product_option
     WHERE merchant_id = 116 AND product_id = p.product_id
     UNION
    SELECT product_id FROM package_2_product
     WHERE merchant_id = 116 AND product_id = p.product_id
  ) THEN 1 ELSE 0 END AS options_exist,
  i.thumbnail,i.thumbnail_width,i.thumbnail_height,
  i.title AS thumbnail_title, i.alt AS thumbnail_alt
FROM 
  product p 
  INNER JOIN category_2_product c2p ON p.merchant_id=c2p.merchant_id 
                                       AND p.product_id=c2p.product_id 
                                       AND c2p.category_id = 84
  LEFT  JOIN product_image        i ON p.merchant_id = i.merchant_id
                                       AND p.product_id = i.product_id
                                       AND i.is_default = 1 
WHERE 
  p.merchant_id = 116
  AND FIND_IN_SET('live',p.param) > 0
  AND FIND_IN_SET('wholesale-only',p.param) = 0
ORDER BY
  p.rank, p.product_name
LIMIT 0, 50;

The pertinent part being the CASE WHEN EXISTS…
is causing the query to take multiple seconds to run. Without the sub-select it finishes in a tenth of a second.

I’m wondering if there is a way to move this type of case, sub-select into an inner join to get the same data, and not have the sub-select run for every row that gets returned.

Attempting the below query with a tweak to the Coalesce line and the LEFT JOIN (SELECT…) p1

I received this error…

Error Code: 1054
Unknown column ‘p1.product_id’ in ‘on clause’

Here is the query…

SELECT SQL_CALC_FOUND_ROWS 
  p.*,
  CASE p.in_stock_msg  WHEN '' THEN 'In stock' ELSE p.in_stock_msg  END AS in_stock_msg,  
  CASE p.out_stock_msg WHEN '' THEN ''         ELSE p.out_stock_msg END AS out_stock_msg,
  COALESCE(p1.product_option_id, p2.product_id) IS NOT NULL AS options_exist,
  i.thumbnail,i.thumbnail_width,i.thumbnail_height,
  i.title AS thumbnail_title, i.alt AS thumbnail_alt
FROM 
  product p 
  INNER JOIN category_2_product c2p ON p.merchant_id=c2p.merchant_id 
                                       AND p.product_id=c2p.product_id 
                                       AND c2p.category_id = 84
  LEFT  JOIN product_image        i ON p.merchant_id = i.merchant_id
                                       AND p.product_id = i.product_id
                                       AND i.is_default = 1 
  LEFT JOIN (
   SELECT product_option_id
     FROM product_option
    WHERE merchant_id = 116
    GROUP BY product_id) p1 on p1.product_id = p.product_id
   LEFT JOIN (
    SELECT product_id
    FROM package_2_product
   WHERE merchant_id = 116
    GROUP BY product_id) p2 on p1.product_id is null and p2.product_id = p.product_id 
WHERE 
  p.merchant_id = 116
  AND FIND_IN_SET('live',p.param) > 0
  AND FIND_IN_SET('wholesale-only',p.param) = 0
ORDER BY
  p.rank, p.product_name

Answer 1

SELECT SQL_CALC_FOUND_ROWS 
  p.*,
  CASE p.in_stock_msg  WHEN '' THEN 'In stock' ELSE p.in_stock_msg  END AS in_stock_msg, 
  CASE p.out_stock_msg WHEN '' THEN ''         ELSE p.out_stock_msg END AS out_stock_msg,
  COALESCE(p1.product_id, p2.product_id) IS NOT NULL AS options_exist,
  i.thumbnail,i.thumbnail_width,i.thumbnail_height,
  i.title AS thumbnail_title, i.alt AS thumbnail_alt
FROM 
  product p 
  INNER JOIN category_2_product c2p ON p.merchant_id=c2p.merchant_id 
                                       AND p.product_id=c2p.product_id 
                                       AND c2p.category_id = 84
  LEFT  JOIN product_image        i ON p.merchant_id = i.merchant_id
                                       AND p.product_id = i.product_id
                                       AND i.is_default = 1 
  LEFT JOIN (
    SELECT product_id
      FROM product_option
     WHERE merchant_id = 116
     GROUP BY product_id) p1 on p1.product_id = p.product_id
  LEFT JOIN (
    SELECT product_id
      FROM package_2_product
     WHERE merchant_id = 116
     GROUP BY product_id) p2 on p1.product_id is null and p2.product_id = p.product_id

WHERE 
  p.merchant_id = 116
  AND FIND_IN_SET('live',p.param) > 0
  AND FIND_IN_SET('wholesale-only',p.param) = 0
ORDER BY
  p.rank, p.product_name

Notes:

• on p1.product_id is null is added to the 2nd left join to prevent it being executed when p1 already has a result.
• The group by clauses prevent the LEFT JOINs from expanding the result set via cartesian product

---

This DDL will create a table with enough fields to show the query working without errors.

create table product_option(merchant_id int, product_id int);
create table package_2_product(merchant_id int, product_id int);
create table category_2_product(merchant_id int, product_id int, category_id int);
create table product_image(merchant_id int, product_id int, is_default int,
    thumbnail int, thumbnail_width int, thumbnail_height int, title int, alt int);
create table product(merchant_id int, product_id int, in_stock_msg int,
    out_stock_msg int, param int, rank int, product_name int);