SQL LEFT JOIN，条件为空白或最后一个条目

Question

我正在尝试编写一个查询，该查询将获取每个资产行并为没有上次审计日期或上次审计日期填写空白（如果该资产存在多个审计）。下面有两个表；第一个用于资产，第二个用于执行的审计。

查询中有两个额外的左连接与另外两个表，但它们是直接的一对一属性映射并且工作正常，正如您在下面的数组结果中看到的那样。

CREATE TABLE `asset_audit` (
    `audit_id` int(16) NOT NULL,
    `audit_asset_id` varchar(16) NOT NULL,
    `audit_date` date NOT NULL,
    `audit_present` varchar(16) NOT NULL,
    `audit_repairs` varchar(16) NOT NULL,
    `audit_comment` varchar(256) NOT NULL
) ENGINE = MyISAM DEFAULT CHARSET = latin1 COLLATE = latin1_swedish_ci;

CREATE TABLE `asset` (
    `asset_id` int(32) NOT NULL,
    `type` varchar(64) NOT NULL,
    `supplier` varchar(16) NOT NULL,
    `asset_name` varchar(64) NOT NULL,
    `asset_desc` varchar(128) NOT NULL,
    `qty` int(32) NOT NULL,
    `serial` varchar(32) NOT NULL,
    `gcyo_id` varchar(32) NOT NULL,
    `purch_date` date NOT NULL,
    `purc_price` varchar(32) NOT NULL,
    `invoice_num` varchar(32) NOT NULL,
    `asset_image` varchar(256) NOT NULL
) ENGINE = MyISAM DEFAULT CHARSET = latin1 COLLATE = latin1_swedish_ci;

INSERT INTO `asset_audit` (`audit_id`, `audit_asset_id`, `audit_date`, `audit_present`, `audit_repairs`, `audit_comment`) VALUES
(1, '1', '2023-03-01', '1', '0', 'this was the second audit of this piano, what a fine piano it is'),
(2, '1', '2023-02-01', '1', '0', 'this is the first audit date'),
(3, '1', '2023-04-04', '1', '0', 'these are the comments'),
(4, '1', '2023-04-04', '1', '1', 'repairs required'),
(5, '1', '2023-04-04', '0', '0', 'not in stock'),
(9, '3', '2023-04-02', '1', '0', 'this is the second asset');

INSERT INTO `asset` (`asset_id`, `type`, `supplier`, `asset_name`, `asset_desc`, `qty`, `serial`, `gcyo_id`, `purch_date`, `purc_price`, `invoice_num`, `asset_image`) VALUES
(1, '1', '1', 'Grand Piano', 'big piano on wheels', 1, '222333', '1001', '2023-04-03', '13,000.65', '1420TY445', 'images/1_Grand Piano.png'),
(3, '9', '3', 'Camera', 'avkans camera', 1, '22233333', '1004', '2023-03-28', '1,200.00', '1420TY44533', '');

这是我一直试图运行的查询的核心。

$sql="SELECT * FROM asset 
  LEFT JOIN asset_type ON asset.type = asset_type.asset_type_id 
  LEFT JOIN suppliers ON asset.supplier = suppliers.supplier_id 
  LEFT JOIN asset_audit ON asset.asset_id = asset_audit.audit_asset_id 
      WHERE
          asset_audit.audit_id = (
          SELECT MAX(audit_id)
          FROM asset_audit
          WHERE asset.asset_id = asset_audit.audit_asset_id
      )
  ORDER BY asset.type ASC, asset.asset_name ASC";

我要查询的代码然后填充一个表（这个页面有一堆其他代码，这是执行这件作品的小节我遇到了麻烦（这是来自 phpize 的片段显示了一个结果）。它应该显示列出的每项资产一行。一项资产尚未进行审计，因此审计日期为空；另一项资产已进行多次审计，因此应显示最后一次审计日期。

mysqli_result Object
(
    [current_field] => 0
    [field_count] => 31
    [lengths] => 
    [num_rows] => 2
    [type] => 0
)

这是结果数组。它仅包含一个响应，并且该响应的最后审核日期是正确的，但数组中没有第二个资产的响应。我试过为第二个资产添加审计条目，但没有为第二个资产设置审计条目；它有同样的问题，它只带回第一个资产的数据。

Array
(
    [asset_id] => 1
    [type] => 1
    [supplier] => 1
    [asset_name] => Grand Piano
    [asset_desc] => big piano on wheels
    [qty] => 1
    [serial] => 222333
    [gcyo_id] => 1001
    [purch_date] => 2023-04-03
    [purc_price] => 13,000.65
    [invoice_num] => 1420TY445
    [asset_image] => images/1_Grand Piano.png
    [asset_type_id] => 1
    [asset_type] => Percussion
    [asset_sub_type] => Tuned Percussion
    [supplier_id] => 1
    [supplier_name] => Piano Supplier 
    [supplier_add_line_1] => line 1
    [supplier_add_line_2] => line 2
    [supplier_suburb] => suburb
    [supplier_postcode] => 2321
    [supplier_state] => state
    [supplier_phone] => 2034956857
    [supplier_contact_name] => Peter supplier
    [supplier_notes] => this is /  some %notes# about"" the  'man
    [audit_id] => 8
    [audit_asset_id] => 1
    [audit_date] => 2023-04-04
    [audit_present] => 1
    [audit_repairs] => 0
    [audit_comment] => this was the second audit of this piano, what a fine piano it is
)

Answer 1

解决了这个问题（对于任何想知道如何解决这个问题的人）

在最后一个 LEFT JOIN 的 WHERE 子句中添加了一个 OR 语句。 （WHERE AND OR 仅在这种情况下适用于 LEFT JOIN）。

SELECT * FROM asset 
                    LEFT JOIN asset_type ON asset.type = asset_type.asset_type_id 
                    LEFT JOIN suppliers ON asset.supplier = suppliers.supplier_id 
                    LEFT JOIN asset_audit ON asset.asset_id = asset_audit.audit_asset_id 
                        WHERE
                                asset_audit.audit_id = (
                                    SELECT MAX(asset_audit.audit_id)
                                    FROM asset_audit
                                    WHERE asset.asset_id = asset_audit.audit_asset_id
                                )
                        OR      asset_audit.audit_id IS NULL
                       
                    ORDER BY asset.type ASC, asset.asset_name ASC