MySQL “Unknown Column in On Clause” [duplicate]

Answer 1

because your mix join syntax

From Mysql[docs]

However, the precedence of the comma operator is less than of INNER JOIN, CROSS JOIN, LEFT JOIN, and so on. If you mix comma joins with the other join types when there is a join condition, an error of the form Unknown column 'col_name' in 'on clause' may occur. Information about dealing with this problem is given later in this section.

the solution is:

To allow the join to be processed, group the first two tables explicitly with parentheses so that the operands for the ON clause are (t1,t2) and t3:

SELECT * FROM (t1, t2) JOIN t3 ON (t1.i1 = t3.i3);

Alternatively, avoid the use of the comma operator and use JOIN instead:

SELECT * FROM t1 JOIN t2 JOIN t3 ON (t1.i1 = t3.i3);

Answer 2

is it that semi colon you have ON posts.author=users.id;

Answer 3

It simply says that author does not exist in post table. either a spelling mistake??

Answer 4

Giving an alias to your tables in from clause and surrounding them with parentheses would make it work. But mixing joins is still a bad practice.