Question

这是我的第一个查询

SELECT 
    tax_productid,
    CONCAT(GROUP_CONCAT(tax_code SEPARATOR " "), " ", sum(tax_iva), "%") AS tax
FROM tb_tax t group by tax_productid order by tax_productid, prd_type desc;

返回适用于产品的税（多种税种可适用于单品）

输出数据，如。

tax_productid       tax
=========================
1                   tax1 tax2 10%
2                   tax1 tax2 20%
3                   tax1 tax2 30%

现在有另一个表product，查询就像

SELECT * FROM product ORDER BY productName;

返回10条产品记录。

我想获得产品明智的税单，但如果没有税收显示为空

SELECT 
    t1.*, 
    CONCAT(GROUP_CONCAT(tax_code SEPARATOR " "), " ", sum(tax_iva), "%") AS tax 
FROM 
    product t1, tb_tax t2 
WHERE
    t2.tax_product = t1.id;

此查询仅返回3行。如果表tb_tax没有产品条目然后返回null，我想列出所有产品。所以我怎么能实现这个目标。

Answer 1

您的查询正在使用INNER JOIN，在这种情况下您应该使用LEFT JOIN：

SELECT 
    t1.*, 
    t2.tax    
FROM 
    product t1
LEFT JOIN (
    SELECT 
      tax_productid,
      CONCAT(GROUP_CONCAT(tax_code SEPARATOR " "), " ", sum(tax_iva), "%") AS tax
    FROM tb_tax 
    GROUP BY tax_productid 
) t2 ON t2.tax_productid = t1.id

Answer 2

使用LEFT JOIN。

SELECT 
    t1.*,
    CONCAT(GROUP_CONCAT(tax_code SEPARATOR " "), " ", sum(tax_iva), "%") AS tax 
FROM 
    product t1
LEFT JOIN 
    tb_tax t2
ON 
    t2.tax_product = t1.id;

Mysql选择查询数据加入

2 个答案: