Question

我有以下表格：

table_categories - id, name, imagepath
table_subcategories - sub_categories_id, sub_category_name
table_subcategory_categories - subcategory_id, category_id (Mapping table for category and sub category)

我想以这种格式获取所有类别及其各自的子类别：

[
    {
        "subcategories": [
            {
                "scname": "Sugar",
                "scid": "5"
            },
            {
                "scname": "Salt",
                "scid": "6"
            },
            {
                "scname": "Jaggery",
                "scid": "7"
            }
        ],
        "name": "Salt and Sugar",
        "id": "1",
        "image": "/images/salt_sugar.png"
    },
    {
        "subcategories": [
            {
                "scname": "Tea",
                "scid": "8"
            },
            {
                "scname": "Coffee",
                "scid": "9"
            },
            {
                "scname": "Tea Bags",
                "scid": "162"
            }
        ],
        "name": "Tea and Coffee",
        "id": "2",
        "image": "/images/tea_and_coffee.png"
    }
]

这是我获取类别和子类别的逻辑。

my $sql_query = "SELECT * from table_categories";
my $statement = $db_handle->prepare ($sql_query) or die "Couldn't prepare query '$sql_query': $DBI::errstr\n";  
$statement->execute() or die "SQL Error: $DBI::errstr\n";  

my @loop_data = ();
while (my @data = $statement->fetchrow_array()) {
    my $sql = "SELECT SCC.subcategory_id, SC.sub_category_name
            FROM
            table_subcategory_categories as SCC
            INNER JOIN 
            table_subcategories as SC
            ON
            SCC.subcategory_id = SC.sub_categories_id
            where SCC.category_id = '$data[0]' ";
    my $stmt = $db_handle->prepare ($sql) or die "Couldn't prepare query '$sql_query': $DBI::errstr\n";  
    $stmt->execute() or die "SQL Error: $DBI::errstr\n";  

    my @InnerLoopData = ();
    while (my @InnerData = $stmt->fetchrow_array()) {
        my %InnerData = ('scid', $InnerData[0], 'scname', $InnerData[1]);
        push(@InnerLoopData, \%InnerData);
    }

    my %data = ('id', $data[0], 'name', $data[1], 'image', $data[2], 'subcategories', \@InnerLoopData);
    push(@loop_data, \%data);
}

my $json_text = to_json(\@loop_data);
print $json_text;
$db_handle->disconnect;

每件事都按预期工作，但问题是因为内部查询响应太慢。由于我有超过1000个类别，内部查询执行超过1000次。

可以请任何人帮我如何优化此查询??

Answer 1

您可以在perl脚本中进行处理，减少数据库调用次数。数据库调用很昂贵。

以排序方式获取表类别列表。

SELECT * FROM table_categories ORDER BY id ASC 将值存储在多维数组中。说类别

获取数组中每个类别的计数。再次排序。保存在数组中说category_count

按category_id顺序选择count（*）FROM table_subcategory_categories group by class_id ASC

获取按类别排序的整个子类别列表保存在多维数组中说sub_categories

SELECT sc。*，scc.category_id来自table_subcategories sc，table_subcategory_categories scc WHERE sc.sub_categories_id = scc.sub_categories_id ORDER BY scc.category_id ASC

然后

my @loop_data = ();
my $currentPos = 0;
my $noOfElementsInThisCategory = 0;
foreach my $i (0 .. $#categories){
     $noOfElementsInThisCategory = $category_count[$i];
     my @InnerLoopData = ();
   for (my $k = $currentPos; $noOfElementsInThisCategory > 0;      $noOfElementsInThisCategory++) {
       my %InnerData = ('scid', $sub_categories [$k][0], 'scname',  $sub_categories [$k][1]);
       push(@InnerLoopData, \%InnerData);
       $currentPos = $currentPos + 1;
}
my %data = ('id', $categories[$i][0], 'name', $categories[$i][1], 'image', $categories[$i][2], 'subcategories', \@InnerLoopData);
    push(@loop_data, \%data);

}
my $json_text = to_json(\@loop_data);
print $json_text;

Answer 2

SCC.subcategory_id
SC.sub_categories_id
SCC.category_id

这个字段应该是索引，然后它会很快。联接应该在索引字段

上

你也不需要（用于获取子类别数据）这里的循环（在循环中查询 - 坏主意），使用单个查询：

SELECT SCC.subcategory_id, SC.sub_category_name, C.*
            FROM
            table_subcategory_categories as SCC
            INNER JOIN 
            table_subcategories as SC
            ON
            SCC.subcategory_id = SC.sub_categories_id
INNER JOIN table_categories as C ON SCC.category_id = C.id

Answer 3

您应该使用placeholders and bind variables在循环外准备查询。

您还应该使用{ RaiseError => 1}创建数据库句柄，这样您就不必明确检查错误。

您还应该将结果作为hashref获取，并避免每次循环时都创建自己的哈希值。

my $dbh  = DBI->connect($dsn, { RaiseError => 1 });

my $sql1 = "SELECT * from table_categories";
my $sth1  = $dbh->prepare ($sql1);  
$sth1->execute();  

my @loop_data = ();

my $sql2 = qq/
    SELECT SCC.subcategory_id as scid, SC.sub_category_name as scname
    FROM table_subcategory_categories SCC
    INNER JOIN table_subcategories SC
      ON SCC.subcategory_id = SC.sub_categories_id
    WHERE SCC.category_id = ?
/;
my $sth2 = $dbh->prepare( $sql2 );

while (my @data = $sth1->fetchrow_array()) {
    $sth2->execute( $data[0] );

    my @InnerLoopData = ();

    while (my $InnerData = $sth2->fetchrow_hashref()) {
        push(@InnerLoopData, $InnerData);
    }

    my %data = ( 'id'            => $data[0],
                 'name'          => $data[1],
                 'image'         => $data[2],
                 'subcategories' => \@InnerLoopData );

    push( @loop_data, \%data );
}

my $json_text = to_json(\@loop_data);
print $json_text;
$dbh->disconnect;

如何避免Mysql中的内部查询

3 个答案: