Question

$STH_1 = $DBH_R->query("SELECT table_name                        
                         FROM information_schema.tables                        
                        WHERE table_name                        
                         LIKE 'v_c_ei\_9%'
                         ");
$stmts_1 = array();

while (($row_1 = $STH_1 -> fetch_assoc()) !== null){

$table_name = $row_1['table_name'];
$stmts_1[] = sprintf("SELECT *  
                        FROM $table_name
                       WHERE (ei_code = '1117') AND (DAY(date_time) != '00') 

                     "); 

} 

$stmt_1 = implode("\nUNION\n", $stmts_1);  
$stmt_1 .= "\nORDER BY date_time ASC";  

$STH_5_2 = $DBH_R->query($stmt_1);

while (($row_5_2 = $STH_5_2 -> fetch_assoc()) !== null){

OK, the script above is working ok. But, I want (must) change its functionality. I must do selection before UNION (something like ORDER BY … LIMIT) but I know – this is not working. The solution is to change sprintif into UNION & UNION & UNION … But the problem is – we don’t know how much tables we have.

The table names from the first query looks like

v_c_ei_9001
v_c_ei_9002
..........
v_c_ei_9030
..........
ect.

Than the q: how change the sprintif (and maybe array $stmts_1 too) from abowe into the UNION in situation when we don’t know how much tables we have?
thnks

Answer 1

When you add ORDER BY and LIMIT clauses to each constituent of the UNION, as follows:

while (($row_1 = $STH_1 -> fetch_assoc()) !== null){

$table_name = $row_1['table_name'];
$stmts_1[] = sprintf("SELECT *  
                      FROM $table_name
                      WHERE (ei_code = '1117') AND (DAY(date_time) != '00')
                      ORDER BY date_time DESC LIMIT 1 
                     "); 

} 

And then you do the following:

$stmt_1 = implode("\nUNION\n", $stmts_1);  
$stmt_1 .= "\nORDER BY date_time ASC";                   // <---- remember this bit

The resulting string in $stmt_1 will look something like this:

SELECT *
FROM v_c_ei_9001
WHERE (ei_code = '1117') AND (DAY(date_time) != '00')
ORDER BY date_time DESC LIMIT 1
UNION
...
UNION
SELECT *
FROM v_c_ei_9030
WHERE (ei_code = '1117') AND (DAY(date_time) != '00')
ORDER BY date_time DESC LIMIT 1
ORDER BY date_time ASC                                 -- it's appearing down here!

As you can see, the final constituent of your UNION has two ORDER BY clauses, which is a syntax error. Either remove the line commented remember this bit above, or else (if you need to reorder the unified queries) you must make the UNION a subquery to an outer query that performs the reordering:

$stmt_1 = "SELECT * FROM ("
$stmt_1 = implode("\nUNION\n", $stmts_1);  
$stmt_1 .= ") ORDER BY date_time ASC";