sql - Is a query with table separated by comma a cross join query? -


i know of sql but, use join, left, cross , on, in query tables separated comma. it's looks cross join me. don't know how test (the result same tries made).

select a.id, b.id a,b a.id_b = b.id

even in great question (with great answers) "what difference between left, right, outer , inner joins?" didn't find answer this.

it would cross join if there wasn't where clause relating 2 tables. in case it's functionally equivalent inner join (matching records id_rel , id)

it's older syntax joining tables still supported in systems, join syntax largely preferred.


Comments

Post a Comment

Popular posts from this blog

javascript - jquery or ashx not working -

i using ajax file upload control. it creating html: <form action="/pages/handlers/asyncfileuploadhandler.ashx" method="post" enctype="multipart/form-data" target="ajaxuploader-iframe-1397144545"> <input type="hidden" name="type" value="mindvoice"> <input type="file" id="imagefile" style="display: block; width: 80%; margin-right: auto; margin-left: auto;"> </form> and target is: <iframe width="0" height="0" style="display:none;" name="ajaxuploader-iframe-1397144545" id="ajaxuploader-iframe-1397144545"></iframe> the form submitted using .submit() . my ashx handler: <%@ webhandler language="c#" class="asyncfileuploadhandler" %> using system; using system.web; using nettpals.core; public class asyncfileuploadhandler : ihttphandler { public void processre...
Read more

opencv - DataType<cv::detail::deriv_type>::depth what is it used for -

i wondering why line used in lucas kanade tracker in opencv: datatype<cv::detail::deriv_type>::depth can explain me? in opencv, depth of mat refers type of data contained in mat 's data buffer. represented integer values correspond given data type. these integers commonly abstracted appropriate macro definition (e.g. uchar data represented macro cv_8u ). cv::datatype type-traits class provides method obtain corresponding integer value without having memorize macro means data type. there few cases user code needs use datatype::depth . more common datatype::type . a simple example shows 1 possible use of datatype::depth : cv::mat uchar_data = cv::mat::ones(3, 3, cv_8uc1); cv::mat float_data; uchar_data.convertto(float_data, cv::datatype<float>::depth); // ^^ equivalently replaced // cv_32f macro float_data.at<float>(0,1) += 0.5f; std::cout << float_data << std::endl;...
Read more

python 3.x - Mapping specific letters onto a list of words -

can please suggest approach write piece of codes automatically maps letter in letter_str onto dic_key (a dict key string type contains dashes match length of words in word_lst)? so, mapping occurs if letter appears in every words in list @ same position no matter how many words in list. if no letter appears @ position words in word list new_dic_key '----'. please see examples below thanks word_lst = ['ague', 'bute', 'byre', 'came', 'case', 'doze'] dic_key = '----' letters_str ='abcdefghijklmnopqrstuvwxyz' new_dic_key = '---e' if word_list = ['bute', 'byre'] new_dic_key = 'b--e' or word_list = ['drek', 'drew', 'dyes'] new_dic_key = 'd-e-' if words in word_list of same length code give want: word_list = ['drek', 'drew', 'dyes'] cols = [] in range(len(word_list[0])): cols.append([]) word in ...
Read more