An SQL join clause combines records from two or more tables or views.

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join performance, index vs. index+foreignkey

I'm using an orm framework that handles data (and its integrity) on top of mysql. So far I ignored foreign keys because the orm handles that just fine. I recently read that foreign keys can enhance ...
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105 views

Relation between lossy decomposition and Natural join?

I am confused with the lossy decomposition and natural join. Natural join(with common attribute): only respective common attribute for the resultant table is formed. Lossy: Natural join on tables ...
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423 views

How can the LEFT JOIN return more results than the unjoined query?

Consider the following queries: mysql> SELECT count(*) FROM list l WHERE l.source='blink'; +----------+ | count(*) | +----------+ | 3372 | +----------+ 1 row in set (0.00 sec) mysql> ...
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1answer
746 views

Calling another function inside a postgres function

I have a function A and function B. Function A Name is f_a() and the code is: Select id,address from A; Function B Name is f_b() code is: Select C.address,B.name From (Select id,name from B, ...
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102 views

Duplicates while doing multiple joins

I have 4 MySQL tables 1. companies - {id*, company} 2. employees - {id*, fname, lname, id_company*} 3. technologies - {id*, technology} 4. company_technologies - {id_technology*, id_company*} Field ...
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129 views

Mysql list data from left join query

I have 2 table and i want to pull data from both of them. This is sample for my table structure: Table tbl_a = id, time, note, customer Table tbl_b = id, time, name, surname, note This is my mysql ...
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99 views

Join and Count based on Specific User ID in SQL

I have the following tables, I inserted some dummy data in it for demonstration purposes. I would like to join these two tables in a way that based on the user ID, I can get the count of similar ...
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1answer
205 views

INSERT INTO selected columns creates new column from JOIN [closed]

I have an INSERT INTO ... SELECT.. query that uses a JOIN on two tables. The results of the query should take a subset of the information and insert it into an existing table. That part works, but the ...
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97 views

Merge two tables on partly matching column

I have below 2 tables. old_name old_value Tom1 100 Kate1 80 Jim1 70 new_name new_value Tom2 70 Kate2 100 Jim2 80 I want to make a new table look like ...
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1answer
284 views

Is it better to put filters on JOIN clauses instead of WHERE clauses

MySQL 5.5.8: Question 1: Is there any difference in the following statements and if so, which is preferred performance wise? Can this even be answered without knowing the row counts of the tables ...
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1answer
120 views

Update , Select and Join using 3 tables

There are 3 tables: Label (Label_ID,Label_Name), Label_ID is Primary Key. Tracks (Prefix, Label_ID), Prefix is Primary Key,Label_ID is empty column Top100 (Prefix,Label) No keys in this table. My ...
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3k views

SQL join query to show rows with non-existent rows in one table

I am trying to get some reporting done for employee time records. We have two tables specifically for this question. Employees are listed in the Members table and each day they enter time entries of ...
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1answer
34 views

Sorting by a field in child table

There are two tables: contract (with primary key contractid) and contractupdate (with field contractid referring to a contract). contractupdate also has DATE field startdate and TINYINT field day. I ...
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1answer
26 views

Optimised query to select document and possible annotations

I have a database of small documents which users can view and annotate. Their annotations are not public. I'm looking to optimise a single query to read the document and annotations for the current ...
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2answers
1k views

How can row estimates be improved in order to reduce chances of spills to tempdb

I notice that when there are spill to tempdb events (causing slow queries) that often the row estimates are way off for a particular join. I've seen spill events occur with merge and hash joins and ...
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1answer
129 views

Query_cache doesn't work with join

I have a simple join query, but for some reason query_cache won't cache it! SELECT id, news, approve, FIXED, DATE, allow_main FROM post LEFT JOIN post_plus ON post.id = post_plus.news_id WHERE ...
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92 views

Can't get rid of filesort on joined query sorting on second table

For some reason, this query: EXPLAIN SELECT * FROM biz as b INNER JOIN listings as l ON b.id = l.biz_id WHERE l.pub_id = 14 AND b.cat_id=310 ORDER BY l.level DESC, l.random DESC LIMIT 5; says it ...
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3k views

MySQL not using indexes when joining against another table

I have two tables, the first table contains all articles / blog posts within a CMS. Some of these articles may also appear in a magazine, in which case they have a foreign key relationship with ...
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1answer
6k views

Syntax of INNER JOIN nested inside OUTER JOIN vs. query results

TLDR; If you look at the 2 execution plans, is there an easy answer to which is better? I purposefully did NOT create indexes so it's easier to see what's happening. Following up on my previous ...
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1answer
41 views

How to create new column with value from other table

I have a table called 'menus': menu_id | sort | page_id | parent_page_id ---------|------|---------|---------------- 1 | 0 | 1 | NULL 1 | 1 | 2 | 1 1 | 2 | ...
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2answers
1k views

Does using LIMIT improve the performance and is it noticeable?

I want to understand the following. Assume that I have a complicated query with let's say a join of 5 tables a group by summations and order by. Letting aside any optimizations to the query itself ...
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750 views

multiple child records in a single row by join

I have one master table (teacher) structured like teacherId Name Class 1 Praveen 10 2 John 9 and having a child table (student) structured like studentId ...
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190 views

Slow join behaviour with 'or' in predicate

I'm faced with a situation which I can't understand and overcome. In short we have left-join query like: select from a left join b on a.key1=b.key1 or a.key1=b.key2 This works VERY slowly, while ...
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1answer
114 views

How to prevent Cartesian joins when value can be in either field?

I have a table of articles where the same article may be presented to different audiences with slightly altered text depending on the user's access level. When this occurs, the article will have the ...
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524 views

MySql INNER JOIN doesn't use composite index

I have two tables, table A which contains date and name of table B entry and table B which contains its name and type. Table a has a composite index on table_b_name and date. I'm selecting all table A ...
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2k views

Conditional Join - Specific Column Select

I have tables like these: Table Employee ( Emp_Id, Full_Name, ...) Table Order ( Submitter_Id, Recipient_Id, ...) Then say that I have conditional join like this: select isnull(Emp.Full_Name, ...
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119 views

Need to create combinations of Parents/Children/Dogs, but can't get past 2nd level of CTE

I've hit a brick wall and please ask help. I'm building a table up for (in this example) a ticketing system where e.g. Ticket 1 will allow Parent: Either parent Peter or Sally, Child: to come without ...
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1answer
66 views

COUNT WITH JOIN [closed]

I've the following tables: fss_post(post_id,text) fss_comment(idComment, post_id, text) Is possible to write a query that give me as result a row for each post_id with total comment count for ...
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1answer
157 views

Get unmatched rows for foreign key

Consider two tables: CREATE TABLE Users( id INT PRIMARY KEY, name VARCHAR(128) ) Engine=InnoDB CREATE TABLE Tags( id INT PRIMARY KEY, user REFERENCES Users(id), tag VARCHAR(128) ) ...
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3answers
863 views

inner joins with where clause for no values?

I have 2 tables : Table1 Parent(varchar) Child (varchar) Table2 C1(varchar) PC (varchar) Sample data: Requirement - I need Table2.C1 values for which column Table2.PC = Table1.Child , ...
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1answer
85 views

Performance: order of tables in joined statement

I have the following SQL statement, running on a SQLite database on a Windows mobile device. SELECT table1.uniqueidentifier1, table1.int1, table1.varchar1, table1.decimal1, table1.decimal2 ...
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48 views

Append columns with values from has many reletionship

I have two tables orders and items. Every Order has many Items. orders - id - created_at - paid_at items - id - title - amount - quantity - order_id Let say that there are 3 kinds of items. Chair, ...
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745 views

Query with PIVOT fails with second LEFT OUTER JOIN to same table

I have a big query that contains a PIVOT and several LEFT OUTER JOINS. I need to add a second LEFT OUTER JOIN to a table that is already joined in the query. When I do this the query errors and I'm ...
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1answer
4k views

Query to compare two subsets of data from the same table?

My problem: I'm trying to compare to subsets of data within one table, and I've got two methods that partially work and one certainty that there's got to be a more correct way to do it. The idea ...
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1answer
1k views

Optimizing multi-table left joins

I have four tables which I am trying to join together. Table event +------------+------------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | ...
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1answer
290 views

MySQL : LEFT JOIN not fetching required data

I'm using left join in MySQL as mentioned below: SELECT Q.QUESTION_ID, school_id FROM Q_MASTER AS Q LEFT JOIN ( SELECT keyword, school_id FROM ...
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2answers
1k views

how to join table from different database with different Users of database?

I have Two database called. Database1: host: 127.0.0.1 user: root1 pass: root1 dbname: db1 Table 1: Category ID INT(11) Primary Key Name VARCHAR(255) Database2: host: 127.0.0.2 ...
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1answer
199 views

Updating a local table with a per-row count(*) which is an aggregate of inner joins on remote server

I have a table of around 350m rows on a linked server in which I added an additional INT column to serve as a count(external_identification) of records as the result of a join on PACKAGE and DOC2. ...
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271 views

How to increase MySQL performance with joins

I have a MySQL database with InnoDB engine. Particularly one of my queries became very slow. The query is shown below: SELECT t1.`ARTREF1` FROM `lijnen` AS t3 INNER JOIN (`kortingen` AS t2, ...
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1answer
2k views

Concatenate one-to-many field in single result?

Say I have the following query: SELECT * FROM AppDetails, AppTags WHERE AppDetails.AppID = '1' AND AppDetails.AppID = AppTags.AppID Which gives the following results: AppID AppName ...
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1answer
59 views

What ways are there to limit the processor / ram requirements of a specific query?

Every once in a while, I need to run some very expensive query on a production server. Think spaghetti joins. This is likely to affect the server operation, and I don't want that. What ways are there ...
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1answer
1k views

Getting multiplication of column values on a joined table based on condition

The title was best I could find to explain my question, I don't think it helps though. Anyways, I have two tables: CREATE TABLE YEARLY_RATES ( RATE_YEAR NUMBER(4,0), RATE NUMBER(6,4) ); ...
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1answer
156 views

Combining multiple left join for one single line

We have a query as below. SELECT tblSData.header, tblEAlert.eMessage FROM tblSData LEFT JOIN tblEAlert ON tblSData.sDataID=tblEAlert.sDataID Where tblSData.aID=".$aID." Order By tblSData.dateTimer ...
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2answers
298 views

Left join not working like expected

I need to do a table join to find missing translations in a translation system. Let's say we have the following simplified table (id is the identifier of the string, the actual translation is not ...
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0answers
91 views

How to index for a group by with columns from different tables?

Is it possible to create an index from columns of 2 different tables? If not, how do we optimize a query that does a group by on columns of 2 different tables? For example: the following ...
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4answers
635 views

Pick one single row each time from either table?

I will run a query and get a list of aID and based on that I will try to match below tables.For each aID I just want one record either from tblGAlert or tblEAlert based on gDateTime and eDateTime ...
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1answer
571 views

How can I join two tables while filtering the results with a“ WHERE IN” clause? [closed]

What I am trying to do is set up a selection filter for a cascading list on my site. When I say selection filter, I mean I am trying to limit, and sort the results based on what the user selects. I ...
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1answer
98 views

MySQL MATCH vs JOIN

I have a lot of data to filter and therefore I wonder is it better to create somekind of indexes and save them to one table and use MATCH or LIKE to find matches or should I use many different tables ...
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2answers
331 views

Distinct rows from a join

I have two MySQL tables: ╔═══════════════════╗ ║ wpss_question_set ║ ╠═══════════════════╣ ║ id ║ ║ quesset ║ ╚═══════════════════╝ ╔════════════════╗ ║ wpss_questions ║ ...
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1answer
2k views

update statement with self join

I needed to copy information from one row of a table to another. I noticed I could do the following: update address set col1 = a2.col1, col2 = a2.col2, .....etc from address a1, address a2 where ...