Join usage analysis of CI frame database query _php example

This article illustrates the join usage of the CI Framework database query. Share to everyone for your reference, specific as follows:

Use each ID in table A to query the information for this ID in the People table. Statement as follows:

$this->db->from (' A ');
$this->db->join (' B ', ' sites.id = b.ID ');

Use each ID in table A to query the information in table B for this ID.

Note the SQL Conventions, and if a column name is duplicated in two tables, you need to precede the column name with a table name and a "." Resolution So sites.id at the location table, the table that contains the ID is sites. When making a SQL multiple table query, it is best to identify the column names uniquely so that you can avoid ambiguity or make yourself understood.

such as: You execute the following statement

$this->db->select (' * ');
$this->db->from (' blogs ');
$this->db->join (' comments ', ' comments.id = Blogs.id ');
$query = $this->db->get ();

Equivalent to executing this SQL statement

SELECT * FROM blogs JOIN comments on comments.id = Blogs.id

If you want to use multiple connections in a query, you can call this function multiple times.

If you need to specify the type of JOIN, you can specify it by using the third argument of this function. Optional options include: left, right, outer, inner, left outer, and right outer.

$this->db->join (' comments ', ' comments.id = Blogs.id ', ' left ');
Build: Left JOIN comments on comments.id = Blogs.id

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