Mysql – Handling large DB tables that have millions of records

Here is the query you need:

SELECT name FROM
(
    SELECT name FROM table1
    UNION
    SELECT name FROM table2
) A;

Here is some sample code based on your question:

use test
drop table if exists table1;
drop table if exists table2;
create table table1
(
    id int not null auto_increment,
    name varchar(10),
    primary key (id)
);
create table table2 like table1;
insert into table1 (name) values ('A'),('B'),('C');
insert into table2 (name) values ('C'),('D'),('E');
SELECT name FROM
(
    SELECT name FROM table1
    UNION
    SELECT name FROM table2
) A;

Here is the execution of that sample code:

mysql> drop table if exists table1;
Query OK, 0 rows affected (0.03 sec)

mysql> drop table if exists table2;
Query OK, 0 rows affected (0.03 sec)

mysql> create table table1 (
    ->     id int not null auto_increment,
    ->     name varchar(10),
    ->     primary key (id)
    -> );
Query OK, 0 rows affected (0.05 sec)

mysql> create table table2 like table1;
Query OK, 0 rows affected (0.06 sec)

mysql> insert into table1 (name) values ('A'),('B'),('C');
Query OK, 3 rows affected (0.06 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> insert into table2 (name) values ('C'),('D'),('E');
Query OK, 3 rows affected (0.11 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> SELECT name FROM (SELECT name FROM table1
    -> UNION SELECT name FROM table2) A;
+------+
| name |
+------+
| A    |
| B    |
| C    |
| D    |
| E    |
+------+
5 rows in set (0.00 sec)

mysql>

Give it a Try !!!

There are many ways to do this. Most common are with a LEFT JOIN / IS NULL check, NOT IN or NOT EXISTS subquery. Here's a solution with the 3rd option:

SELECT t2.id, t2.name 
FROM table2 AS t2
WHERE NOT EXISTS
      ( SELECT *
        FROM table3 AS t3
        WHERE t3.table1_id = @t1_id     -- the t1.id value (choice) that you want to check
          AND t3.table2_id = t2.id
      ) ;