Use "select up to 1 rows" only if you are sure that all the records returned will have the same value for the field(s) you are interested in. If not, you will be reading only the first record which matches the criteria, but may be the second or the third record has the value you are looking for.
what is the difference of select single and select up to one row in abap
原文：http://blog.csdn.net/champaignwolf/article/details/45332971