MySQL選擇查詢數據加入

Question

這是我的第一個查詢

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;

會傳回適用於產品的適當稅款（多種稅種可能適用於單一產品）

輸出數據一樣。

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

現在有另一個表product ，查詢就像

SELECT * FROM product ORDER BY productName;

返回10條產品記錄。

我想獲取明智的產品稅單，但如果沒有稅，請顯示null

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;

該查詢僅返回3行。 如果表tb_tax沒有產品條目，那么我想列出所有產品，然后返回null。 所以我怎么能做到這一點。

Answer 1

您的查詢使用INNER JOIN ，在這種情況下，您應該使用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;