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How to Use "Union" In Codeigniter Query Builder?

4 minutes read

In CodeIgniter query builder, the "union" clause can be used to combine the results of two or more SELECT statements into a single result set. This can be useful when you want to merge data from multiple tables or conditions in a single query.


To use "union" in CodeIgniter query builder, you can simply call the union() method after each SELECT statement. For example:


$this->db->select('column1, column2'); $this->db->from('table1');


$this->db->union();


$this->db->select('column3, column4'); $this->db->from('table2');


$query = $this->db->get();


This will generate a SQL query that combines the results of the two SELECT statements into a single result set. The union() method can be called multiple times to combine more than two SELECT statements if needed.


It's important to note that the SELECT statements used with the union() method must have the same number of columns and data types in order to work properly.


How to use "union" with joins in codeigniter query builder?

In CodeIgniter's query builder, you can use the union() method to combine the results of two or more queries using the UNION operator. Here's how you can use union() with joins in a CodeIgniter query:

  1. Start by setting up your initial query with the select(), from(), and join() methods as needed:
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$query1 = $this->db->select('column1, column2')
                   ->from('table1')
                   ->join('table2', 'table1.id = table2.id', 'inner');


  1. Next, set up your second query in a similar way:
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$query2 = $this->db->select('column3, column4')
                   ->from('table3')
                   ->join('table4', 'table3.id = table4.id', 'inner');


  1. Finally, combine the two queries using the union() method:
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$query = $query1->union($query2)
                ->get();


This will generate a SQL query that combines the results of query1 and query2 using the UNION operator. You can then execute the query and retrieve the results as needed.


Keep in mind that the two queries being combined must have the same number of columns and compatible data types in order for the UNION operation to work correctly.


How to control the order of execution of queries in a "union" statement in codeigniter query builder?

In Codeigniter query builder, you can control the order of execution of queries in a "union" statement by using subqueries. Here's an example:

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$this->db->select('column1, column2')
         ->from('table1')
         ->where('condition1', 1);

$subquery = $this->db->select('column1, column2')
                     ->from('table2')
                     ->where('condition2', 2)
                     ->get_compiled_select();

$this->db->select('column1, column2')
         ->from('table3')
         ->where('condition3', 3)
         ->union($subquery);

$query = $this->db->get();


In the above example, we first build the main query for table1 and then create a subquery for table2. We then include the subquery in the union statement of the main query. This way, we can control the order of execution of the queries and ensure that the subquery is executed before the main query.


By using subqueries in this way, you can control the order of execution of queries in a "union" statement in Codeigniter query builder.


How to alias columns in a "union" query in codeigniter query builder?

To alias columns in a "union" query in CodeIgniter query builder, you can use the select() method with an alias for each column in each query of the union. Here's an example:

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$query1 = $this->db->select('column1 as alias1, column2 as alias2, column3')->from('table1')->get_compiled_select();
$query2 = $this->db->select('column1 as alias1, column2 as alias2, column3')->from('table2')->get_compiled_select();

$union_query = $this->db->query($query1 . ' UNION ' . $query2);

foreach ($union_query->result() as $row) {
    echo $row->alias1;
    echo $row->alias2;
    echo $row->column3;
}


In this example, we are selecting columns from two different tables with column aliases in each query. We then combine the two queries using the UNION operator and execute the final query to get the results with the specified column aliases.


What is the behavior of "union" when used with aggregate functions in codeigniter query builder?

In CodeIgniter query builder, when the "union" method is used with aggregate functions such as SUM, COUNT, AVG, etc., the resulting query will combine the results of the two select statements using the UNION operator.


For example, if you have two select statements with aggregate functions like this:

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$this->db->select('SUM(amount) as total_income');
$this->db->from('payments');
$this->db->where('status', 'completed');

$this->db->union();

$this->db->select('SUM(amount) as total_expense');
$this->db->from('expenses');
$this->db->where('category', 'utilities');


The resulting query will look like this:

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SELECT SUM(amount) as total_income FROM payments WHERE status = 'completed'
UNION
SELECT SUM(amount) as total_expense FROM expenses WHERE category = 'utilities'


This will combine the results of the two select statements into a single result set.


How to include the keyword "union" in a codeigniter query?

You can include the keyword "UNION" in a CodeIgniter query by using the query builder library provided by CodeIgniter. Here is an example of how you can include the keyword "UNION" in a CodeIgniter query:

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$this->db->select('*');
$this->db->from('table1');
$this->db->where('some_column', 'some_value');

$this->db->union();

$this->db->select('*');
$this->db->from('table2');
$this->db->where('some_other_column', 'some_other_value');

$query = $this->db->get();

return $query->result();


In the above example, we first select all columns from table1 where the value of some_column is equal to 'some_value'. Then, we use the union() method to add the "UNION" keyword to the query. Finally, we select all columns from table2 where the value of some_other_column is equal to 'some_other_value'.


This will generate a SQL query with the "UNION" keyword included, allowing you to combine the results of the two queries.

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