Recover city from database id

Question

I’m getting information from a select, which pulls the states and cities registered in the database. In the value of each select (city and state) is their id in the table (only this way to work with the function in AJAX). But at the time of entering I’m not managing to turn that state and city id into the name of the city (select * from cidade where id = $_post[cidade]). By placing a mysqli_query in the PHP variable and then try to insert it gives an error.

$cidade = $_POST['cidade'];
        $estado = $_POST['estado'];

        //Validando dados do formulário
        if(!empty($nome) && !empty($endereco)  && !empty($num) && !empty($CEP) && !empty($bairro) && !empty($cidade) && !empty($estado)){
            //Caso todos os campos sejam preenchidos, insere os valores na tabela estabelecimento
            mysqli_query($conectar, "INSERT INTO estabelecimento VALUES (NULL, '$nome', '$CNPJ', '$endereco',
            '$num', '$comp', '$CEP', '$bairro', '$cidade', '$estado', 0)");

The first two records were entered manually into the database.

Answer 1

There is an error in your insertion:

Note this example the syntax should contain the name of the fields you will insert,

mysqli_query($con,"INSERT INTO pessoa ($FirstName,$LastName,$Age) 
VALUES ('Glenn','Quagmire',33)");

Something else in case of routines you need before the Insert effect a select you can do so :

mysqli_query($con,"INSERT INTO pessoa ($FirstName,$LastName,$Age) 
SELECT 'Glenn','Quagmire',33 FROM pessoa WHERE id=12";

This avoids server resource consumption because it minimizes the number of queries, the Insert and the select within the same consultation.

Would that be your relationship between entities:

SELECT * FROM establishment and INNER JOIN cities ON e.cod_city = cities.id_city;

That is to say :

SELECT ALL FIELDS OF TABLES ESTABLISHMENT AND CITIES WHERE cod_city (known as foreign key FK of another table) is equal to primary key id_city of the table cities.

Same thing for table states....