MySQL left connection SQL left-hand join statement detailed

MySQL tutorial left-connect SQL left-hand join statement detailed
MySQL left-join query is a way of federated query, that is, two related tables are joined together in this way to query, which makes it easier to call data and avoid multiple loops nesting.

Left JOIN keyword LEFT join for query
, the written explanations are as follows:

Left OUTER join (left connection): The result set includes both matching rows of the join table and all rows of the left join table.
Let me explain, that is to say, the return result of the left-linked query contains all the data on the left table of the left-hand join keyword, as well as the eligible resources in the right table that matches the left table. Simply put, the left table as the center, with the right table in accordance with the conditions of resources.

Syntax for SQL statements:

Sql:select a.a, A.B, A.C, B.C, B.D, B.f from a left-out join B on a.a = B.C

I believe you will be very dizzy after reading, it does not matter, I explain that we all understand, seemingly complex, in fact, organized very clearly. What it means is:

One
Select A.A, A.B
In
A.A is the meaning of a field in Table A.

Two
Between select and from Is the field in table B of table a related to query.

Three
From a left out join B
is from table A as the main left link on the right table B,
The match condition is the a field of a table equals the C field of table B.

Through these explanations I believe that we all understand that 80%, the following we through the example of the demonstration again to say:

First set up two tables

Table pic
Table to store the name of the picture

CREATE table ' image '. ' Pic ' (

' id ' int (ten) NOT NULL Auto_increment primary key,
' pname ' varchar character set UTF8 collate utf8_unicode_ci NOT NULL

) engine = InnoDB

Table II for storing picture comments

CREATE table ' image '. ' Comment ' (

' id ' int (ten) NOT NULL Auto_increment primary key,
' PID ' int (a) NOT NULL,
' content ' varchar character Set UTF8 collate utf8_unicode_ci NOT NULL

) engine = InnoDB

Next I want to combine the two tables through a SQL statement, note that before we query two tables must be the first to write a table of SQL and then in accordance with the criteria loop query another table, now do not, through this query, the return results into an array,

SQL statement wording: SELECT
pic.*,comment.*
From
Pic Left JOIN Comment
On
Pic.id=comment.pid

The code is as follows:

$conn = mysql_connect ("localhost", "root", "");

if (! $conn) {

Die (' Connect MySQL database tutorial failed: '. Mysql_error () ');

}

mysql_select_db (' image ');

$sql = "Select
pic.*,comment.*
From
Pic Left JOIN Comment
On
Pic.id=comment.pid ";

$re =mysql_query ($sql);

while ($re 1=mysql_fetch_array ($re))

{

$arr []= $re 1;

}

Print_r ($arr);

Print the results as follows:

Array

(

[0] => Array

(

[0] => 1

[ID] => 1

[1] => AAAAAAAAA

[PName] => aaaaaaaaa

[2] => 1

[3] => 1

[PID] => 1

[4] => CCCCCCC

[Content] => CCCCCCC

)

[1] => array

(

[0] => 2

[ID] => 2

[1] => bbbbbbbbbbb

[PName] => bbbbbbbbbbb

[2] => 2

[3] => 2

[PID] => 2

[4] => VVVVVVV

[Content] => VVVVVVV

)

)

Ok