database - MySQL Limit with Many to Many Relationship -

-

given schema implementing tags

item itemid, itemcontent

tag tagid, tagname

item_tag itemid, tagid

what best way limit number of items return when selecting tags?

select i.itemcontent, t.tagname item  inner join itemtag on i.id = it.itemid  inner join tag t on t.id = it.tagid

is of course easiest way them back, using limit clause breaks down, because duplicate of items each tag, counts toward number of rows in limit.

my second solution uses mysql function group_concat() combine tags matching item comma-separated string in result set.

select i.itemcontent, group_concat(t.tagname order t.tagname) taglist item    inner join itemtag on i.id = it.itemid    inner join tag t on t.id = it.tagid group i.itemid;

the group_concat() function mysql feature, it's not part of standard sql.


Comments

Post a Comment

Popular posts from this blog

c++ - How do I get a multi line tooltip in MFC -

-
right now, have tool tip pops when hover on edit box. problem tool tip contains multiple error messages , in 1 long line. need have each error message on own line. error messages contained in cstring new line seperating them. my existing code below. bool ontooltiptext(uint, nmhdr* pnmhdr, lresult* presult) { assert(pnmhdr->code == ttn_needtexta || pnmhdr->code == ttn_needtextw); // need handle both ansi , unicode versions of message tooltiptexta* pttta = (tooltiptexta*)pnmhdr; tooltiptextw* ptttw = (tooltiptextw*)pnmhdr; // tchar szfulltext[256]; cstring strtiptext=_t(""); uint nid = pnmhdr->idfrom; if (pnmhdr->code == ttn_needtexta && (pttta->uflags & ttf_idishwnd) || pnmhdr->code == ttn_needtextw && (ptttw->uflags & ttf_idishwnd)) { // idfrom hwnd of tool nid = ::getdlgctrlid((hwnd)nid); } //m_errprojaccel[ch] contains 1 or more error messages each...
Read more

asp.net - In javascript how to find the height and width -

-
i having .aspx , masterpage page. in masterpage loading aspx page . in aspx page design using generating table structure using code-behind server in javascript how find height , width . regards, kumar if you're looking table height , width, should it var tbl = document.getelementbyid('tbldemo'); alert(tbl.offsetwidth + '\n' + tbl.offsetheight);
Read more

c# - DataTable to EnumerableRowCollection -

-
i have datatable according client requirement can contain invalid date "00/00/1999",null,empty string.(typed datset) when collect them enumerables wish convert of invalid forms empty string. (i.e) enumerablerowcollection<datarow> query = order in _table.asenumerable() select new { orderdate= string.isnullorempty(convert.tostring(order.field<datetime>("orderdate")) || (how check date invalid date) ? string.empty : order.field<datetime>("orderdate") } how check whether date valid date or not? datetime.tryparse...like given current code: datetime date; enumerablerowcollection<datarow> query = order in _table.asenumerable() select new { orderdate= string.isnullorempty(convert.tostring(order.field<datetime>("orderdate")) || !datetime.tryparse(convert.tostring(order.field<datetime...
Read more