如何将多个表的foreach限制为一个

I have the issue with LIMIT with foreach using PHP.

Basics: I have 50 different tables and in every table I have 2 rows.

When I try to add LIMIT 1 to $$modules_for_all, then I see 50 rows, but I want to see only 1. If I add LIMIT 2, then I see 100 rows.

How I can connect all these tables as a one LIMIT 1 to get 1 row in foreach?

<?php
for ($i = 1; $i <= 50; $i++) {

  // $array_table_name contains names with tables
  $table_names = $array_table_name[$i];

  $modules_for_all = 'g_module_for_all_'.$i;
  $$modules_for_all = $db->QueryFetchArrayAll("SELECT * FROM $table_names WHERE user='1' LIMIT 1");

}

for ($i = 1; $i <= 50; $i++) {

  $modules_for_from = ${"g_module_for_all_$i"};

  foreach ($modules_for_from as $m_foreach_as) {

    echo $m_foreach_as['id'];

  }

}

Example tables:

table_1
id   date_added
1    2018-12-01 00:00:00
2    2018-12-02 00:00:00

table_2
id   date_added
1    2018-12-03 00:00:00
2    2018-12-04 00:00:00

table_3
id   date_added
1    2018-12-05 00:00:00
2    2018-12-06 00:00:00

Example foreach:

<?php
$array_table_name_1 = 'table_1';
$array_table_name_2 = 'table_2';
$array_table_name_3 = 'table_3';

$for_table_1 = $db->QueryFetchArrayAll("SELECT * FROM $array_table_name_1 WHERE id='1' LIMIT 1 ORDER BY date_added");
$for_table_2 = $db->QueryFetchArrayAll("SELECT * FROM $array_table_name_2 WHERE id='1' LIMIT 1 ORDER BY date_added");
$for_table_3 = $db->QueryFetchArrayAll("SELECT * FROM $array_table_name_3 WHERE id='1' LIMIT 1 ORDER BY date_added");

foreach ($for_table_1 as $m_foreach_as) {
  echo $m_foreach_as['id'];
}

foreach ($for_table_2 as $m_foreach_as) {
  echo $m_foreach_as['id'];
}

foreach ($for_table_3 as $m_foreach_as) {
  echo $m_foreach_as['id'];
}

// Now result is '111' but I want only '1' (realted to make LIMIT 1 to all foreach)

The only way to connect the tables is by using a UNION. So you will need to build one large UNION query and then perform the select after the loop:

$tables = array();
for ($i = 1; $i <= 50; $i++) {  
  // $array_table_name contains names with tables
  $table_names = $array_table_name[$i];
  $tables[] = "(SELECT * FROM $table_names WHERE user='1')";    
}
$query = implode(" UNION ", $tables) . " ORDER BY date_added LIMIT 1";
$result = $db->QueryFetchArrayAll($query);
foreach ($result as $row) {
    echo $row;
}

You are gonna have to use UNION ALL for summing this rows together before ordering and limiting the results.

But keep in mind that a query like this will only work if all the tables in your array have the same structure. If they do not, then you will have to be specific in the query to make them have the same fields.

$array_table_name = [
    'table_1',
    'table_2',
    'table_3',
];

$search_id = 1;

$selectsArray = [];
foreach ($array_table_name as $table_name) {
    $selectsArray[] = "SELECT * FROM $table_name WHERE id='$search_id'
";
}

As you see, I am using foreach() and not for() so you won't update the for by decreasing/increasing the number of tables in the array. So to finally have:

$selectsUnion = implode("UNION ALL
", $selectsArray) . "ORDER BY date_added 
LIMIT 1";

You can see the code tested and query mounted here: https://3v4l.org/HXH2K

I solved my problem using this: foreach ($modules_for_from as $m_foreach_as) if ($tmp++ < 1) {