union (chain operation 12)

Union usage example:

Db::field('name')
    ->table('think_user_0')
    ->union('SELECT name FROM think_user_1')
    ->union('SELECT name FROM think_user_2')
    ->select();

Closure usage:

Db::field('name')
    ->table('think_user_0')
    ->union(function ($query) {
        $query->field('name')->table('think_user_1');
    })
    ->union(function ($query) {
        $query->field('name')->table('think_user_2');
    })
    ->select();

or

Db::field('name')
    ->table('think_user_0')
    ->union([
        'SELECT name FROM think_user_1',
        'SELECT name FROM think_user_2',
    ])
    ->select();

supports UNION ALL operation, for example:

Db::field('name')
    ->table('think_user_0')
    ->unionAll('SELECT name FROM think_user_1')
    ->unionAll('SELECT name FROM think_user_2')
    ->select();

or

Db::field('name')
    ->table('think_user_0')
    ->union(['SELECT name FROM think_user_1', 'SELECT name FROM think_user_2'], true)
    ->select();

Each union method is equivalent to an independent SELECT statement.

The SELECT statements inside UNION must have the same number of columns. Columns must also have similar data types. Also, the order of the columns in each SELECT statement must be the same.