php - 即使使用 DISTINCT，SQL SELECT 查询也会回显两次

Question

我正在 PHP 中创建一个简单的 SQL 查询 - 出于某种原因，即使DISTINCT使用它，它也会像这样显示两次：

BC
BC
OH
OH
TX
TX

这是我的代码：

<?php
$sql = "SELECT DISTINCT `title`,`extra_fields_search` FROM `blahblah_items` WHERE catid=336 ORDER BY `blahblah_items`.`extra_fields_search` ASC ";
$partnerlisting= mysql_query($sql) or die(mysql_error());
while($row = mysql_fetch_assoc($partnerlisting))
foreach($row as $cname => $cvalue){
echo '<li>'.substr($row[extra_fields_search], 0, 2).'</li><br>';
}
;

?>

我怎样才能使它只打印出一个？

Answer 1

SELECT 
    `title`,
    `extra_fields_search` 
FROM 
    `blahblah_items` 
WHERE 
    catid=336 
GROUP BY 
    SUBSTRING(extra_fields_search,1,2) # here. group by first two characters of extra_fields_search
    # or just "GROUP BY extra_fields_search", depends what you need
ORDER BY 
    `blahblah_items`.`extra_fields_search` ASC

SELECT DISTINCT a,b FROM table工作方式与SELECT a,b FROM table GROUP BY a,b

请遵循文档：

• http://dev.mysql.com/doc/refman/5.0/en/distinct-optimization.html
• http://dev.mysql.com/doc/refman/5.0/en/group-by-functions.html

Answer 2

试试这个

SELECT `title`,`extra_fields_search` 
FROM `blahblah_items` WHERE catid=336 
Group BY extra_fields_search
ORDER BY `blahblah_items`.`extra_fields_search` ASC

编辑：在你的代码中试试这个

    while($row = mysql_fetch_assoc($partnerlisting))
    $rows[] = $row;
    foreach($rows as $row){
    echo '<li>'.substr($row[extra_fields_search], 0, 2).'</li><br>';
    }
   ;

Answer 3

Distinct 确保您获得唯一的行。它不能确保您将获得唯一的列值。在您的情况下，如果您考虑结果中的所有字段，您会注意到每一行与另一行至少有一个字段不同。

所以你永远不会得到

Col1  Col2
A     B
A     B

但是你可以得到

Col1  Col2
A     B
A     C

Answer 4

尝试：

SELECT `title`, group_concat(distinct `extra_fields_search`)
FROM `blahblah_items` 
WHERE catid=336 
Group BY `title`
ORDER BY 2