An acronym for "Structured Query Language" (it's not the name of a specific DBMS product), SQL can be used in relational database management systems (RDBMS) to query, update, delete, and insert data as well as modify the structure of the database. It can also be used to manage schemas and data ...

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For a MySQL query, how do you determine physical and logical I/O?

I'm used to having a showplan in Microsoft SQL and have gotten lazy with using tools to tune MySQL queries. How can I determine physical/logical I/O for a query in MySQL? (EXPLAIN shows the index ...
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SQL Calculated Field in both SELECT and GROUP BY clause

Often in querying my MS SQL Server databases, I need to create a calculated field, such as this (CASE WHEN A.type = 'Workover' THEN 'Workover' ELSE (CASE WHEN substring(C.category, 2, 1) = 'D' ...
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318 views

selecting where two columns are in a set

This might be a silly question, and my suspicion is that I can't do this, but is there a construct in SQL that would allow me to do something like the following: SELECT whatever WHERE col1,col2 IN ...
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Why are NULLs sorted first?

Why is it that when we have a NULL value in a column and we order by the value ascending, the NULLs are sorted first? select 1 as test union all select 2 union all select NULL union all select 3 ...
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IN and NOT IN for XML column

I have a table with a xml column. Xml is similar to <Root> <Row> <user>abc</user> <Rowid>1</Rowid> </Row> <Row> ...
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Algorithm for finding the longest prefix

I have two tables. First one is a table with prefixes code name price 343 ek1 10 3435 nt 4 3432 ek2 2 Second is call records with phone numbers number time 834353212 10 ...
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Why do we have SHRINKDATABASE command in SQL Server

I have read across the internet where Finest Developer says shrinkdatabse should be avoided as it causes increase in fragmentation and decrease in performance . So i would like to know from you guys ...
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Deleting all duplicates

I am trying to delete all the duplicates but keeping single record only (shorter id). Following query deletes duplicates but take lot of iterations to delete all copies and keeping original ones. ...
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How to Best Implement nearest neighbour search in mysql?

So, in short, What should be the data type of latitude and longitude? What SQL command I should call to get the first 100 nearest restaurants for example? Detail: I have 100k biz record each with ...
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Is it possible to limit the maximum disk usage for a user

Is it possible to set a limit on the amount of physical disk space a user (or group of users) can use within a single database? For example is there some method of associating a schema with a ...
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SQL Server 2008 R2 High CPU

We are having a SQL Server R2 Web Edition database, based on Windows Server 2008 R2 with a strong CPU and 32GB RAM DDR3. Our database works very hard (we run around 600,000 queries in 1 minute) and ...
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Oracle's left join and where clauses errors

CREATE TABLE "ATABLE1" ( "COLUMN1" VARCHAR2(20 BYTE), "COLUMN2" VARCHAR2(20 BYTE) ); CREATE TABLE "ATABLE2" ( "COLUMN1" VARCHAR2(20 BYTE), "COLUMN2" VARCHAR2(20 BYTE) ); ...
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Database behind a Multilingual User Interface

This question is about an issue somewhat more complicated than the one which has already been addressed in these old questions, all of which are duplicates of one another: Suggestion for database ...
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How to express this constraint in a database schema?

I have the following functional dependencies which are in BCNF: a,b -> c a -> d b -> d With the additional constraint, that no a and b should be combined with a c, where a and b have ...
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1answer
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Deleting large amounts (4 million rows) of mysql data efficiently, on a very regular basis

We have a mysql table that at any given time has about 12 million rows. We need to delete old data to keep the size of the table somewhat manageable. Right now we are running this query daily, at ...
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Database size - MDF too large?

I'm maintaining a SQL Server 2005 database which hosts approximately 2.9Tb of data (2 x 1.45Tb - I have a RAW schema and an ANALYSIS schema so basically two copies of the data ingested). The recovery ...
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MySQL and window functions

It seems that MySQL does not support window functions. E.g. the simple: COUNT(*) OVER() AS cnt does not work. What I am not sure is if this applies to commercial version as well (I assume the ...
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Oracle 11g: performance improvements of inserts

I have a table of 500 millions of rows (and growing) I did the following to improve performance of inserts: On database side: dropped all indexes and constraints disabled logging On application ...
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PAGE compression in SQL Server effect on running queries

I am planning to apply PAGE compression to some of the big tables in my database (DW to be precise). These tables are fairly big with over 15 Billion rows. When I applied the compression in test ...
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T-SQL - OPTION (FAST x) and trace flag 8722

I have searched a long time and still haven't found the answer to my problem. Our Dynamics AX is generating queries with the query hint OPTION (FAST x), which sometimes forces a bad execution plan to ...
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Efficient query to get greatest value per group from big table

Given the table: Column | Type id | integer latitude | numeric(9,6) longitude | numeric(9,6) ...
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SELECT DISTINCT on one column, while returning the other columns?

I have a query which uses three lookup tables to get all the information I need. I need to have DISTINCT values for one column, however I also need the rest of the data associated with it. My SQL ...
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How do I do a complex GROUP BY in MySQL?

I have a table that contains several keys into other tables (where each key is comprised of multiple columns). I would like to be able to group rows together that have an equal key, but I don't want ...
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Is the optimisation fence behaviour of a CTE (WITH query) specified in the SQL:2008 standard? If so, where?

I see frequent references to WITH queries (common table expressions, or CTEs) acting as an optimisation fence, where the server isn't permitted to push filters down into the CTE queries, pull common ...
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1answer
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sysschedules datatype choices

I am Curious. Why does TSQL save the values in msdb.dbo.sysschedules for date and times as ints instead of using datetime. I assume the reason dates back to something in SQL Server 2000. Was it a ...
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Maximum value from multiple tables

Consider the following situation: we have T1, T2, & T3 tables in which saved id and price of products. Now we need to find the id of products which have maximum price of all 3 tables. I have this ...
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Convert units of measurement

Looking to calculate the most suitable unit of measurement for a list of substances where the substances are given in differing (but compatible) unit volumes. Unit Conversion Table The unit ...
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SQL Server: query performance (search 2 million rows)

Got an interesting one for all of you SQL guru's out there. Now this search is only taking a couple of seconds, but it's quite intensive and there must be a better way. Maybe I'm expecting too much? ...
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Optimize select on subquery with COALESCE(…)

I have a large view that I use from within an application. I think I've narrowed down my performance problem, but I'm unsure how to fix it. A simplified version of the view looks like this: SELECT ...
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Individual queries run on 10ms, with UNION ALL they are taking 290ms+ (7.7M records MySQL DB). How to optimise?

I have a table that stores available appointments for teachers, allowing two kinds of insertions: Hourly based: with total freedom to add unlimited slots per day per teacher (as long as slots don't ...
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Filtering data ordered by rowversion

I have a SQL table of data with the following structure : CREATE TABLE Data( Id uniqueidentifier NOT NULL, Date datetime NOT NULL, Value decimal(20, 10) NULL, RV timestamp NOT NULL, ...
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1answer
545 views

computed columns, index, clustered index and covering index?

We all know the we can only have one clustered view per table, cool. But apparently you don't have values of computed columns on leaf even when using a clustered index and they are computed each time. ...
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1answer
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Friends relations in MySQL

I'm developing a friendship relation in MySQL where friend relation is mutual. If A is friend of B, then B is friend of A. If one of the users end friendship then relation drops. I want to learn which ...
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selecting more fields causes catastrophic failure

I have a connection to a linked server from sql server 2012 to pervasive sql. When I do select field1, field2, field3 from mytable everything works! However, when I do select field1, field2, field3, ...
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Unexplained InnoDB timeouts

I've been seeing some very basic updates timing out lately and have not been able to determine the cause. An example: //# Query_time: 51 Lock_time: 0 Rows_sent: 0 Rows_examined: 0 UPDATE photos ...
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1answer
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Designing Simple Schema for Disaggregation of Demand Forecast

I am doing a simple database design task as a training exercise where I have to come up with a basic schema design for the following case: I have a parent-child hierarchy of products (example, Raw ...
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2answers
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Different results rebuilding an index online and offline

I have a non-clustered, non-unique index on a foreign key column of type bigint. When I rebuild the index online, the average fragmentation drops to 3%, with 2 fragments, and 30 pages. When I run ...
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Which SQL standard is most broadly and completely supported?

I want to concentrate on one of the SQL language specs for learning and reference purposes. Which version of the SQL standard is most broadly supported across DBMS'es so that I can focus my time and ...
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4answers
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Is a join optimized to a where clause at runtime?

When I write a query like this... select * from table1 t1 join table2 t2 on t1.id = t2.id Does the SQL optimizer, not sure if that is the correct term, translate that to... select * from table1 ...
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In Oracle how do I save a sequence.nextval in a variable to be reused in multiple inserts?

I'm writing a script to populate some tables with data for testing. I would like to write something like the following but I don't know how to do it (I'm Oracle 11g) SET ENABLED_USER_ID = ...
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Why can't I use a CASE statement to see if a column exists and not SELECT from it?

Why does something like this not work? SELECT CASE WHEN NULLIF(COL_LENGTH('Customers', 'Somecol'), '') IS NULL THEN NULL ELSE Somecol END AS MyTest FROM Customers; I am just checking if the column ...
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3answers
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Passing in the table into a stored procedure

Is it possible to pass in the name of a table into a stored procedure? For example, suppose you have several views of the same table. They all have the exact same structure. You want a store ...
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Do you know an easy way to generate one record for each hour of the past 12 hours?

I have a report that shows the count of events for the past 12 hours, grouped by the hour. Sounds easy enough, but what I am struggling with is how to include records that cover the gaps. Here is an ...
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2answers
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Is there any difference between putting a column alias at the start or the end of the column definition?

I've always seen and written my column aliases as SELECT 1 as ColumnName but today came across a query that used SELECT ColumnName = 1 Is there any difference in how these two queries get ...
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3answers
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Are individual queries faster than joins?

Conceptual question: Are individual queries faster than joins, or: Should I try to squeeze every info I want on the client side into one SELECT statement or just use as many as seems convenient? ...
6
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1answer
271 views

how do I insert a default row?

If I create a table with an identity column as primary key, and all the other columns have default values, for example create table rr (id int identity(1,1) primary key, dt datetime default ...
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SQL Server 2008 mirrored Database with manual failover, need reporting database

Our current setup is a principal database used in production with a manual failover mirrored database for HA. I need to setup a reporting database as well. What would be the best approach? Shall I ...
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attach/detach v.s. backup/restore

I am using SQL Server 2008 Enterprise. I need to transfer the database (as a whole) to another server (to make a duplicate database to setup another test environment). I have two choices, (1) making ...
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Combine column from multiple rows into single row

I've got some customer_comments split out into multiple rows due to database design, and for a report I need to combine the comments from each unique id into one row. I previously tried something ...
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3answers
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SQL Server Primary key / clustered index design decision

Looking for some advice regarding a table / index design decision I've got to make on some tables that I've got to port into SQL server from an existing 4GL based database. I've got a product history ...