Query a database with results from multiple tables?

Question

There are some similar questions around but they aren\'t quite what I\'m looking for, so forgive me if you think this is answered elsewhere.

I am basically looking

Answer 1

Generic example (in PHP):

Constructing dynamic SQL or building your SQL queries with the aid of a programming language would look like this (in PHP for ex.):

$pdos = $pdo->query("SHOW TABLES LIKE '%_name'");
$tables = $pdos->fetchAll();

$query = 'SELECT * FROM '.implode(' UNION SELECT * FROM ');
$pdo->query($query);

The fetchAll method will return an array containing the names of each table selected.

The implode($glue, $array) function takes an array and concatenates every value in the array using the $glue parameter - usually you take an array of values and implode them using $glue = ',' to create a coma separated list of values.

In our case the implode has a partial query as $glue in order to create one big UNION JOIN query.

Once the final $query is build it should look something like:

SELECT * FROM table_1_name
    UNION
SELECT * FROM table_2_name
    UNION
SELECT * FROM table_3_name
    ....
    ....
    UNION
SELECT * FROM table_4000_name

The result should contain all of the DISTINCT rows from all 4000 tables.

Specific example (in SQL-only format):

SELECT    GROUP_CONCAT(
              CONCAT('select * from ', table_name)
              SEPARATOR ' union '
          )
    INTO  @my_variable
    FROM  information_schema.tables
    WHERE table_schema = 'dbname'
    AND   table_name LIKE '%_name';

PREPARE   my_statement FROM @my_variable;
EXECUTE   my_statement;

• The first statement will get all of the table names from the information_schema database;
• The CONCAT function prefixes every table name with a a 'SELECT * FROM ' string;
• The GROUP_CONCAT does the job that implode would have done in PHP;

• The INTO clause makes sure the values are saved inside a variable named my_variable;

• The PREPARE statement takes a string value (such as the one you saved in my_variable) and checks if the value is an SQL query;

• The EXECUTE statement takes a "prepared statement" and well... executes it.

@my_variable is a temporary variable but it can only be of a scalar type (varchar, int, date, datetime, binary, float, double etc.) it is not an array.

The GROUP_CONCAT function is an "aggregate function" which means it takes an aggregate value (similar concept to an array - in our case the result set of our query) and outputs a simple string result.