Tiếp theo, hãy nhìn vào HOẶC toán hạng. Dịch sang tiếng Anh đơn giản, OR có nghĩa là một hay khác, tôi không quan tâm mà một trong những '. Điều này có hiệu lực một toán hạng dễ dàng, chỉ có một trong hai điều kiện cần phải được hoàn thành, và nếu cả hai là đúng sự thật thì tốt hơn. | SPATIAL SEARCH 27 Next look at the OR operand. Translated into plain English OR means one or the other I don t care which one . This is in effect an easy-going operand where only one of the two conditions needs to be fulfilled and if both are true then the better. So no matter whether we look at income or literacy as long as either one or both is high the area gets selected. OR operations always result in a maximum number of items to be selected. Somewhat contrary to the way the word is used in everyday English AND does not give us the combination of two criteria but only those records that fulfill both conditions. So in our case only those areas that have both high literacy and high income at the same time are selected. In effect the AND operand acts like a strong filter. We saw this above in the section on conditional queries where all conditions had to be fulfilled. The last example illustrates that we can combine Boolean operations. Here we look for all areas that have a high literacy rate but not high income. It is a combination of our first example NOT HI with the AND operand. The result becomes clear if we rearrange the query to state NOT HI AND HL. We say that AND and OR are binary operands which means they require one descriptor on the left and one on the right side. As in regular algebra parentheses can be used to specify the sequence in which the statement should be interpreted. If there are no parentheses then NOT precedes overrides the other two. 5 Spatial Relationships Spatial relationships are one of the main reasons why one would want to use a GIS. Many of the cartographic characteristics of a GIS can be implemented with a drawing program while the repository function of large spatial databases is often taken care of by traditional database management systems. It is the explicit storage of spatial relationships and or their analysis based on geometric reasoning that distinguishes GIS from the rest of the pack. We ended the last chapter with a .
