带有条件的MySQL LEFT JOIN示例

Question

我正在使用以下查询来检索所需的对象：

SELECT *
FROM tb_po_items
LEFT JOIN tb_materials ON tb_po_items.po_material = tb_materials.id_material
LEFT JOIN tb_services ON tb_po_items.po_service = tb_services.id_service
WHERE po_id =47

现在我需要添加以下内容：

条件1 - ＆gt;如果tb_po_items.mos = 1则LEFT JOIN tb_units ON tb_materials.material_unit = tb_units.id_unit

其他

条件2 - ＆gt;如果tb_po_items.mos = 2则LEFT JOIN tb_units ON tb_services.service_unit = tb_units.id_unit

如何在MySQL查询中实现这两个条件的使用？

提前致谢。

Answer 1

这是实验性的，所以在你上线之前检查一下。

SELECT * FROM tb_po_items
IF(tb_po_items.mos = 1, LEFT JOIN tb_materials ON tb_po_items.po_material = tb_materials.id_material, LEFT JOIN tb_services ON tb_po_items.po_service = tb_services.id_service)
WHERE po_id =47

Answer 2

试试这个：

SELECT *
FROM tb_po_items
LEFT JOIN tb_materials ON tb_po_items.po_material = tb_materials.id_material
LEFT JOIN tb_services ON tb_po_items.po_service = tb_services.id_service
LEFT JOIN tb_units ON (
  (tb_materials.material_unit = tb_units.id_unit AND tb_po_items.mos = 1)
OR
  (tb_services.service_unit = tb_units.id_unit AND tb_po_items.mos = 2) )
WHERE po_id =47;