How to Insert into the table if the record does not exist

Question

How to insert in the table if there is no record, as for example there is the table turma_materia, has the following information:

+----+-------+---------+
| ID | turma | materia |
+----+-------+---------+
| 1  |  1    |  1      |
| 2  |  1    |  2      |
| 3  |  2    |  2      |

If you try to insert Class 1 with Class 1, it will check if you have, if you do not have Index, if you do not have it

Answer 1

You can use a SELECT instead of VALUES in the INSERT:

INSERT INTO turma_materia(ID, turma, materia)
SELECT 1, 1, 1
 WHERE NOT EXISTS (SELECT 1
                     FROM turma_materia
                    WHERE ID = 1
                      AND turma = 1
                      AND materia = 1);

If you only want to set the values once, you can use a subquery in the FROM:

INSERT INTO turma_materia(ID, turma, materia)
SELECT x.ID, x.turma, x.materia
  FROM (SELECT 1 AS ID,
               1 AS turma,
               1 AS materia) x
 WHERE NOT EXISTS (SELECT 1
                     FROM turma_materia tm
                    WHERE tm.ID = x.ID
                      AND tm.turma = x.turma
                      AND tm.materia = x.materia);