---Posts----

id  | title | content |
544 | Alan  | Blah

---Postmeta----

metaid | post_id | meta_key | meta_value
1      |  544    | age      | 45
2      |  544    | email    | test@hotmail.co.uk

SELECT posts.post_id,
       postmeta.age,
       postmeta.email,
FROM
       posts
JOIN posts
ON posts.post_id = postmeta.post_id

SELECT p.id, pm.age, pm.email
FROM posts p
JOIN
(
   SELECT
     post_id,
     MAX(CASE WHEN meta_key = 'age'   THEN  meta_value END) AS age,
     MAX(CASE WHEN meta_key = 'email' THEN meta_value END) AS email
   FROM postsmeta
   GROUP BY post_id
) pm ON p.id = pm.post_id

SELECT
  posts.id,
  postmeta.key,
  postmeta.meta_value
FROM
  posts
JOIN
  postmeta ON posts.id = postmeta.post_id
WHERE
  postmeta.key IN ('age', 'email')

SELECT posts.post_id,
postmeta.age,
postmeta.email,
FROM
posts
JOIN postsmeta
ON posts.post_id = postmeta.post_id

SELECT posts.id,
       postmeta.age,
       postmeta.email,
       m.meta_key,
       m.meta_value
FROM
       posts  p
JOIN Postmeta m
ON p.id = m.post_id

metaid | post_id | age | email
1      |  544    | 45  | test1@hotmail.co.uk
2      |  545    | 51  | test2@hotmail.co.uk
3      |  546    | 20  | test3@hotmail.co.uk
4      |  547    | 26  | test4@hotmail.co.uk

SELECT posts.post_id, postmeta.age, postmeta.email FROM posts JOIN postmeta ON posts.post_id = postmeta.post_id;

SELECT p.id,
       pm1.meta_value AS age,
       pm2.meta_value AS email
FROM
       posts AS p
  LEFT JOIN
       postmeta AS pm1
    ON pm1.post_id = p.id
   AND pm1.meta_key = 'age'

  LEFT JOIN
       postmeta AS pm2
    ON pm2.post_id = p.id
   AND pm2.meta_key = 'email' ;