MySQL 8.0

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CREATE TABLE wp_postmeta (
  `meta_id` INTEGER primary key auto_increment,
    `post_id` INTEGER,
  `meta_key` VARCHAR(31),
  `meta_value` VARCHAR(37)
);

INSERT INTO wp_postmeta
  (`meta_id`, `post_id`, `meta_key`, `meta_value`)
VALUES
  ('6677055', '36828', '_product_version', '4.7.0'),
  ('6677056', '36828', '_price', '439.2'),
  ('6677057', '36828', '_wp_trash_meta_comments_status', 'a:0:{}'),
  ('6677058', '36828', '_wp_desired_post_slug', 'notebook-black-spire-v-nitro-vn7-591g'),
  ('6677059', '36828', '_wxr_import_has_attachment_refs', '1'),
  ('6677060', '36828', '_wp_page_template', 'default'),
  ('6677049', '36828', '_virtual', 'no'),
  ('6677047', '36828', '_backorders', 'no'),
  ('6677048', '36828', '_sold_individually', 'yes'),
  ('6677053', '36828', '_stock', NULL),
  ('6677052', '36828', '_download_expiry', '-1'),
  ('6677050', '36828', '_downloadable', 'no'),
  ('6677042', '36828', '_regular_price', '1596');

Records: 13  Duplicates: 0  Warnings: 0


SELECT `meta_id`, `post_id`, `meta_key`, `meta_value`
  FROM `wp_postmeta` 
  WHERE post_id=36828 and meta_key="_sale_price";


meta_id
post_id
meta_key
meta_value


SELECT
if (exists(SELECT 1 from wp_postmeta WHERE post_id=36828 and meta_key="_sale_price" ) 
, @sql := 'UPDATE `wp_postmeta` 
  SET meta_value = 100
WHERE post_id=36828 and meta_key="_sale_price" ',
@sql := 'INSERT INTO wp_postmeta ( `post_id`, `meta_key`, `meta_value`) VALUES (36828,"_sale_price",100) ');
    PREPARE stmt FROM @sql;
    EXECUTE stmt;
    DEALLOCATE PREPARE stmt;


if (exists(SELECT 1 from wp_postmeta WHERE post_id=36828 and meta_key="_sale_price" ) 
, @sql := 'UPDATE `wp_postmeta` 
  SET meta_value = 100
WHERE post_id=36828 and meta_key="_sale_price" ',
@sql := 'INSERT INTO wp_postmeta ( `post_id`, `meta_key`, `met


INSERT INTO wp_postmeta ( `post_id`, `meta_key`, `meta_value`) VALUES (36828,"_sale_price",100)



Statement prepared


SELECT `meta_id`, `post_id`, `meta_key`, `meta_value`
  FROM `wp_postmeta` 
  WHERE post_id=36828 and meta_key="_sale_price";


meta_id
post_id
meta_key
meta_value


6677061
36828
_sale_price
100