在使用 foreach 循环构建 sql 查询时删除最后一个“UNION ALL”答案

【问题标题】：remove the last "UNION ALL" in building up a sql query with foreach loop在使用 foreach 循环构建 sql 查询时删除最后一个“UNION ALL”
【发布时间】：2014-09-02 09:49:10
【问题描述】：

我想使用 foreach 循环构建以下 SQL 查询字符串

SELECT 'name' AS `field`. GROUP_CONCAT(DISTINCT `name`  ORDER BY `name`) AS `value`
FROM product
UNION ALL
SELECT 'category' AS `field`. GROUP_CONCAT(DISTINCT `category` ORDER BY `category`) AS `value`
FROM product
UNION ALL
SELECT 'year' AS `field`. GROUP_CONCAT(DISTINCT `year` ORDER BY `year`) AS `value`
FROM product

棘手的部分是下面的循环会在year 的最后一个 SELECT 语句中导致不必要的“UNION ALL”。

$selects = array("name"=>"Name","category"=>"Category","year"=>"Year");

$sql="";

foreach($selects as $select => $title)
{
  $sql .= "SELECT '".$select."' AS `field` , GROUP_CONCAT( DISTINCT `".$select."`ORDER BY `".$select."` SEPARATOR '|' ) AS `value` FROM `product` UNION ALL";
}

谁能告诉我如何从数组中的最后一个元素（在本例中为年份）中删除最后一个 UNION ALL？

【问题讨论】：

标签： php mysql sql loops foreach

【解决方案1】：

在您的 foreach 中，创建一个不带 UNION ALL 的查询数组。那么：

$sql = implode(' UNION ALL ', $queries);

【讨论】：

【解决方案2】：

在变量中使用UNION ALL。将其初始化为空字符串。

在每次迭代时添加它。然后将UNION ALL赋值给变量。

$selects = array("name"=>"Name","category"=>"Category","year"=>"Year");

$sql="";
$union_all="";

foreach($selects as $select => $title)
{
  $sql .= $union_all . "SELECT '".$select."' AS `field` , GROUP_CONCAT( DISTINCT `".$select."`ORDER BY `".$select."` SEPARATOR '|' ) AS `value` FROM `product`";
  $union_all=" UNION ALL ";
}

试试看！！！

【讨论】：

【解决方案3】：

下面的代码可以解决你的问题。

$selects = array("name"=>"Name","category"=>"Category","year"=>"Year");

$sql="";

foreach($selects as $select => $title)
{
  $sql .= "SELECT '".$select."' AS `field` , GROUP_CONCAT( DISTINCT `".$select."`ORDER BY `".$select."` SEPARATOR '|' ) AS `value` FROM `product`". ($select!='Year')?" UNION ALL":"";
}

【讨论】：