天道酬勤,学无止境

join

What is the maximum number of table joins in MariaDB?

Is the number of table joins in MariaDB 10 limited to 61 as is the case for MySQL or another number? (I couldn't find the answer in the MariaDB documentation or by googling).

2021-06-21 10:40:04    分类:问答    mysql   sql   join   limit   mariadb

Why is LEFT JOIN slower than INNER JOIN?

I have two queries, the first one (inner join) is super fast, and the second one (left join) is super slow. How do I make the second query fast? EXPLAIN SELECT saved.email FROM saved INNER JOIN finished ON finished.email = saved.email; id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE finished index NULL email 258 NULL 32168 Using index 1 SIMPLE saved ref email email 383 func 1 Using where; Using index EXPLAIN SELECT saved.email FROM saved LEFT JOIN finished ON finished.email = saved.email; id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE

2021-06-21 09:24:41    分类:问答    mysql   join   query-optimization

COUNT(*) with LEFT JOIN and GROUP BY to include NULL in MySQL

I'm trying to get the results from a table including the appearance count in a foreign table. This table can have 0 or more appearances. like in the following example: table: color +------+---------+ | id | name | +------+---------+ | 1 | red | | 2 | blue | | 3 | yellow | | 4 | green | +------+---------+ table: fruit +--------+----------+ | name | color_id | +--------+----------+ | apple | 1 | | banana | 3 | | grape | 4 | | lemon | 3 | +--------+----------+ So I need to list every color and the occurrence in the fruit table, returning something like this: 1, red, 1 2, blue, 0 3, yellow, 2 4

2021-06-15 18:13:21    分类:问答    mysql   join   group-by

Cannot select entity through identification variables without choosing at least one root entity alias

Ads entity is described by geographic information: Country> Region>County. The Ads entity is only linked with County. Consequently, retrieving Ads by countries will require us joining entities twice. My goal is counting the number of Ads for a given country. For that, I tried this DQL query but without success: public function getMotorsAdsCountByCountry($slug){ $qb = $this->_em->createQueryBuilder() ->select("m.id, COUNT(m.id) AS cnt") ->from("MinnAdsBundle:MotorsAds", "m") ->join("m.county","county")->addSelect("county") ->join("county.region","region")->addSelect("region") ->join("region

2021-06-15 16:47:30    分类:问答    symfony   join   doctrine-orm   inner-join   dql

how to do a JOIN FETCH in jpa criteria

I am trying to translate the query below to criteria api. SELECT er from ereturn er JOIN FETCH product_item pi ON pi.ereturn_id = er.id WHERE pi.status = "RECEIVED" To something like this: CriteriaBuilder builder = em.getCriteriaBuilder(); CriteriaQuery<Ereturn> criteria = builder.createQuery( Ereturn.class ); Root<Ereturn> er = criteria.from(Ereturn.class); Join<Ereturn, ProductItem> productItemJoin = er.join("productItems", JoinType.LEFT); Fetch<Ereturn, ProductItem> productItemFetch = er.fetch("productItems", JoinType.LEFT); List<Predicate> predicates = new ArrayList<>(); predicates.add

2021-06-15 14:18:37    分类:问答    hibernate   jpa   join   hibernate-criteria

Need SQL guru for a complex query

I have several tables that I am trying to get some data out of, and I am very close, but cannot quite close the deal. I have the following tables: EVENT USER FRIEND USER__FRIEND EVENT__INVITATION USER and FRIEND are linked via the USER__FRIEND table (which contains a USER_ID and a FRIEND_ID field) EVENT__INVITATION links an EVENT with a FRIEND (it has EVENT_ID and INVITEE_ID) I am trying to get all EVENTS where: I am the EVENT creator ($myUserID = EVENT.CREATOR_ID) or I am invited to the event ($myUserID = EVENT__INVITATION.INVITEE_ID) or one of my FRIENDs is the creator of the EVENT (

2021-06-15 07:03:54    分类:问答    mysql   sql   join

pyspark join rdds by a specific key

I have two rdds that I need to join them together. They look like the followings: RDD1 [(u'2', u'100', 2), (u'1', u'300', 1), (u'1', u'200', 1)] RDD2 [(u'1', u'2'), (u'1', u'3')] My desired output is: [(u'1', u'2', u'100', 2)] So I would like to select those from RDD2 that have the same second value of RDD1. I have tried join and also cartesian and none is working and not getting even close to what I am looking for. I am new to Spark and would appreciate any help from you guys. Thanks

2021-06-15 06:53:27    分类:问答    join   pyspark   rdd

How to calculate new column depending on aggregate function on group using dplyr (add summary statistics on the summary statistics)?

Quite often I need to calculate a new column for an R dataframe (in long form), whose value should depend on an aggregate function (e.g. sum) of a group. For instance, I might want to know what fraction of sales a product accounts for on any given day: daily fraction = revenue for product i on day d / sum or revenue for all products on day d My current strategy is to summarise and join: library(dplyr) join_summary <- function(data, ...) left_join(data, summarise(data, ...)) data = data.frame( day = c(1,1,2,2,3,3), product = rep(c("A", "B"), 3), revenue = c(2, 4, 8, 7, 9, 2) ) data2 <- data %>%

2021-06-15 06:08:02    分类:问答    r   join   dplyr   aggregate   summary

MySQL Left Join (Unknown Column)

I am having problems with my query. MySQL Query: SELECT DISTINCT(`users`.`username`), `users`.`full_name`, `users`.`profile_picture_url`, `users`.`followed_by_count`, `users`.`follows_count`, `users`.`bio`, `users`.`id` FROM `users`,`interests` LEFT JOIN `blocked` ON `blocked`.`receiver_id` = `users`.`id` AND `blocked`.`actor_id` = 100 AND `blocked`.`blocked_reason` = 'Blocked' WHERE `blocked`.`receiver_id` IS NULL AND `users`.`instagram_active` = 1 AND `users`.`banned` = 0 AND `interests`.`user_id` = `users`.`id` AND `interests`.`interest` = 'Food' AND `interests`.`active` = 1 AND `users`

2021-06-15 06:01:23    分类:问答    mysql   sql   join   left-join

Does LINQ use DataRelations to optimize joins?

I can't find the answer to this anywhere, and before I start pawing through generated code with Reflector I thought it'd be worth asking: Suppose I have the following LINQ query run against DataTables in a DataSet: var list = from pr in parentTable.AsEnumerable() join cr in childTable.AsEnumerable() on cr.Field<int>("ParentID") equals pr.Field<int>("ID") where pr.Field<string>("Value") == "foo" select cr; If there's a DataRelation between the parent table and the child table that uses the key fields shown, will LINQ use it? That is, will it find the rows in the parent table for which Value is

2021-06-15 03:30:11    分类:问答    c#   linq   join   dataset   datarelation