How to create a dynamic query using php?

Question

I have a table like this:

// mytable
+----+---------+------------+
| id | id_post | code_table |
+----+---------+------------+
| 1  | 34523   | 1          |
| 2  | 3453    | 4          |
| 3  | 43434   | 2          |
| 4  | 54321   | 1          |
| 5  | NULL    | NULL       |
| 6  | 32411   | 2          |
| 7  | 42313   | 1          |
| 8  | 34242   | 2          |
+----+---------+------------+
//                    ^ all of my focus is on this column

Also I have this array:

$convert_code_name = array (
                        "1" => "Post1", 
                        "2" => "Post2", 
                        "3" => "Post3",
                        "4" => "Post4"  
                           );

Now I want to create this :

$query = "select * from post1
             union all
          select * from post2
             union all
          select * from post4";

          // there isn't "post3", because 3 isn't exist in the code_table column

How can I do that?

---

Here is my try :

// connect to database
$stm = $db->prepare('select * from mytable');
$stm->execute();
$result = $stm->fetch();

/* array_unique: removes duplicate values
   array_filter: removes "NULL" values */

array_filter(array_unique($result[code_table]), function($item) {
    return $item != 'NULL';
});

foreach($item as $numb){
    $query .= 'select * from'.$convert_code_name[$numb].'union all';
}

But I don't know why my code doesn't work, How can I do that?

Answer 1

First, use SELECT DISTINCT in the query to get unique values, so you don't need to call array_unique .

Then, once you have all the values, you can use implode to connect all the SELECT queries with UNION ALL .

$stm = $db->prepare("SELECT DISTINCT code_table FROM mytable WHERE code_table IS NOT NULL");
$stm->execute();
$results = $stm->fetchAll();
// This returns a 2-dimensional array, we just want one column
$results = array_column($results, 'code_table');

$query = implode(' UNION ALL ', array_map(function($code_table) use ($convert_code_name) {
    return "SELECT * FROM " . $convert_code_name[$code_table];
}, $results));

Answer 2

$query .= 'select * from'.$convert_code_name[$numb].'union all';

will generate wrong sql, change it to (I assume that $convert_code_name[$numb] contains full table name like Post1, Post2):

$query = '';
foreach($item as $numb){
    $query .= ($query!=''?' union all ':'') . 'select * from '.$convert_code_name[$numb];
}

Answer 3

You simply need to first execute this query:

SELECT DISTINCT code_table FROM events

Then you can build your second query like from its result, which only contains 1 row for each existing code_table .