insert in sql table using join with no reference matching code example

Example 1: select all if id in another table

SELECT * FROM terms WHERE id IN 
   (SELECT term_id FROM terms_relation WHERE taxonomy = "categ")

Example 2: insert to first table if field A equals field B from a second table using sql

UPDATE Table_1 SET Field_X = (SELECT Field_Y FROM Table_2 WHERE Table_1.Field_A=Table_2.Field_B LIMIT 1)