求助大佬，case when 报错

大佬帮忙解释下这个小白问题，万分感谢。

SELECT FPJE, TKJE, invocie_sales_fan.MV002 FROM invocie_sales_fan
LEFT JOIN return_sales_fan ON invocie_sales_fan.MV002 = return_sales_fan.MV002
不加CASE WHNE 可以看到表

加了CASE WHEN 直接报错

SELECT FPJE, TKJE, invocie_sales_fan.MV002 FROM invocie_sales_fan
LEFT JOIN return_sales_fan ON invocie_sales_fan.MV002 = return_sales_fan.MV002 ,E END