如果没猜错的话,你的pi_image是Blob类型的。blob类型 不支持 SELECT DISTINCT下面是Oracal(其他数据库基本上一样)的帮助文挡,可参考Also, LOBs are not allowed in the following places: LOBs are not allowed in partitioned tables nor are they allowed in clustered tables and thus cannot be a cluster key. LOBs are not allowed in GROUP BY, ORDER BY, SELECT DISTINCT, aggregates and JOINS. However, UNION ALL is allowed on tables with LOBs. UNION, MINUS, and SELECT DISTINCT are allowed on LOB attributes if the object type has a MAP or ORDER function. LOBS are not analyzed in ANALYZE... COMPUTE/ESTIMATE STATISTICS statements. LOBs are not allowed in index only tables. LOBs are not allowed in VARRAYs.
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分别对应,仅限制于integer,string,象blob,image等类型并不支持用UMION
select pi_image from...
union all
select pi_image from...
注意中间加了一个 all ..