查詢操作和性能優(yōu)化
1.基本操作
增
models.Tb1.objects.create(c1='xx', c2='oo') 增加一條數(shù)據(jù),可以接受字典類型數(shù)據(jù) **kwargs obj = models.Tb1(c1='xx', c2='oo')obj.save()
查
models.Tb1.objects.get(id=123) # 獲取單條數(shù)據(jù),不存在則報錯(不建議)models.Tb1.objects.all() # 獲取全部models.Tb1.objects.filter(name='seven') # 獲取指定條件的數(shù)據(jù)models.Tb1.objects.exclude(name='seven') # 獲取指定條件的數(shù)據(jù)
刪
models.Tb1.objects.filter(name='seven').delete() # 刪除指定條件的數(shù)據(jù)
改
models.Tb1.objects.filter(name='seven').update(gender='0') # 將指定條件的數(shù)據(jù)更新,均支持 **kwargsobj = models.Tb1.objects.get(id=1)obj.c1 = '111'obj.save() # 修改單條數(shù)據(jù)
2.Foreign key的使用原因
但是多表查詢會降低速度,大型程序反而不使用外鍵,而是用單表(約束的時候,通過代碼判斷)
extra
extra(self, select=None, where=None, params=None, tables=None, order_by=None, select_params=None) Entry.objects.extra(select={'new_id': "select col from sometable where othercol > %s"}, select_params=(1,)) Entry.objects.extra(where=['headline=%s'], params=['Lennon']) Entry.objects.extra(where=["foo='a' OR bar = 'a'", "baz = 'a'"]) Entry.objects.extra(select={'new_id': "select id from tb where id > %s"}, select_params=(1,), order_by=['-nid'])F查詢
from django.db.models import F models.Tb1.objects.update(num=F('num')+1)Q查詢
方式一:
Q(nid__gt=10) Q(nid=8) | Q(nid__gt=10) Q(Q(nid=8) | Q(nid__gt=10)) & Q(caption='root')
方式二:
con = Q() q1 = Q() q1.connector = 'OR' q1.children.append(('id', 1)) q1.children.append(('id', 10)) q1.children.append(('id', 9)) q2 = Q() q2.connector = 'OR' q2.children.append(('c1', 1)) q2.children.append(('c1', 10)) q2.children.append(('c1', 9)) con.add(q1, 'AND') con.add(q2, 'AND') models.Tb1.objects.filter(con)exclude(self, *args, **kwargs)
# 條件查詢 # 條件可以是:參數(shù),字典,Q
select_related(self, *fields)
性能相關(guān):表之間進行join連表操作,一次性獲取關(guān)聯(lián)的數(shù)據(jù)。
model.tb.objects.all().select_related() model.tb.objects.all().select_related('外鍵字段') model.tb.objects.all().select_related('外鍵字段__外鍵字段')prefetch_related(self, *lookups)
性能相關(guān):多表連表操作時速度會慢,使用其執(zhí)行多次SQL查詢 在內(nèi)存中做關(guān)聯(lián),而不會再做連表查詢
# 第一次 獲取所有用戶表# 第二次 獲取用戶類型表where id in (用戶表中的查到的所有用戶ID)models.UserInfo.objects.prefetch_related('外鍵字段')annotate(self, *args, **kwargs)
# 用于實現(xiàn)聚合group by查詢from django.db.models import Count, Avg, Max, Min, Sum v = models.UserInfo.objects.values('u_id').annotate(uid=Count('u_id'))# SELECT u_id, COUNT(ui) AS `uid` FROM UserInfo GROUP BY u_idv = models.UserInfo.objects.values('u_id').annotate(uid=Count('u_id')).filter(uid__gt=1)# SELECT u_id, COUNT(ui_id) AS `uid` FROM UserInfo GROUP BY u_id having count(u_id) > 1v = models.UserInfo.objects.values('u_id').annotate(uid=Count('u_id',distinct=True)).filter(uid__gt=1)# SELECT u_id, COUNT( DISTINCT ui_id) AS `uid` FROM UserInfo GROUP BY u_id having count(u_id) > 1extra(self, select=None, where=None, params=None, tables=None, order_by=None, select_params=None)
# 構(gòu)造額外的查詢條件或者映射,如:子查詢Entry.objects.extra(select={'new_id': "select col from sometable where othercol > %s"}, select_params=(1,))Entry.objects.extra(where=['headline=%s'], params=['Lennon'])Entry.objects.extra(where=["foo='a' OR bar = 'a'", "baz = 'a'"])Entry.objects.extra(select={'new_id': "select id from tb where id > %s"}, select_params=(1,), order_by=['-nid'])reverse(self):
# 倒序 models.UserInfo.objects.all().order_by('-nid').reverse()# 注:如果存在order_by,reverse則是倒序,如果多個排序則一一倒序下面兩個 取到的是對象,并且注意 取到的對象可以 獲取其他字段(這樣會再去查找該字段降低性能
defer(self, *fields):
models.UserInfo.objects.defer('username','id')或models.UserInfo.objects.filter(...).defer('username','id')# 映射中排除某列數(shù)據(jù)only(self, *fields):
# 僅取某個表中的數(shù)據(jù)models.UserInfo.objects.only('username','id')或models.UserInfo.objects.filter(...).only('username','id') 以上就是本文的全部內(nèi)容,希望對大家的學(xué)習(xí)有所幫助,也希望大家多多支持VEVB武林網(wǎng)?! ?/p>
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