I have two tables that I am joining with the following query...
select *
from Partners p
inner join OrganizationMembers om on p.ParID = om.OrganizationId
where om.EmailAddress = 'my_email#address.com'
and om.deleted = 0
Which works great but some of the columns from Partners I want to be replaced with similarly named columns from OrganizationMembers. The number of columns I want to replace in the joined table are very few, shouldn't be more than 3.
It is possible to get the result I want by selectively choosing the columns I want in the resulting join like so...
select om.MemberID,
p.ParID,
p.Levelz,
p.encryptedSecureToken,
p.PartnerGroupName,
om.EmailAddress,
om.FirstName,
om.LastName
from Partners p
inner join OrganizationMembers om on p.ParID = om.OrganizationId
where om.EmailAddress = 'my_email#address.com'
and om.deleted = 0
But this creates a very long sequence of select p.a, p.b, p.c, p.d, ... etc ... which I am trying to avoid.
In summary I am trying to get several columns from the Partners table and up to 3 columns from the OrganizationMembers table without having a long column specification sequence at the beginning of the query. Is it possible or am I just dreaming?
select om.MemberID as mem
Use th AS keyword. This is called aliasing.
You are dreaming in your implementation.
Also, as a best practice, select * is something that is typically frowned upon by DBA's.
If you want to limit the results or change anything you must explicitly name the results, as a potential "stop gap you could do something like this.
SELECT p.*, om.MemberId, etc..
But this ONLY works if you want ALL columns from the first table, and then selected items.
Try this:
p.*,
om.EmailAddress,
om.FirstName,
om.LastName
You should never use * though. Always specifying the columns you actually need makes it easier to find out what happens.
But this creates a very long sequence
of select p.a, p.b, p.c, p.d, ... etc
... which I am trying to avoid.
Don't avoid it. Embrace it!
There are lots of reasons why it's best practice to explicity list the desired columns.
It's easier to do searches for where a particular column is being used.
The behavior of the query is more obvious to someone who is trying to maintain it.
Adding a column to the table won't automatically change the behavior of your query.
Removing a column from the table will break your query earlier, making bugs appear closer to the source, and easier to find and fix.
And anything that uses the query is going to have to list all the columns anyway, so there's no point being lazy about it!
Related
I have recently started to learn oracle, and I am having difficulty understanding this inner join on the tables.
INSERT INTO temp_bill_pay_ft
SELECT DISTINCT
ft.ft_id,
ft.ft_credit_acct_no,
ft.ft_debit_acct_no,
ft.ft_stmt_nos,
ft.ft_debit_their_ref,
ft.ft_date_time
FROM
funds_transfer_his ft
INNER JOIN temp_bill_pay_lwday_pl dt
ON ft.ft_id = dt.ac_ste_trans_reference || ';1'
AND ft.ft_credit_acct_no = dt.ac_id;
It is this line specifically which I dont understand, why do we use || here, I suppose it is for concatenation.
ON ft.ft_id = dt.ac_ste_trans_reference||';1'
Can somebody please explain to me this sql query. I would really appreciate it. Thank you.
This is string concatenation. The need is because there is a design error in the database and join keys are not the same in the two tables. So the data might look something like this:
ft_id ac_ste_trans_reference
123;1 123
abc;1 abc
In order for the join to work, the keys need to match. One possibility is to remove the last two characters from ft_id, but I'm guessing those are meaningful.
I can speculate on why this is so. One possibility is that ft_id is really a compound key combined into a single column -- and the 1 is used to indicate the "type" of key. If so, then there are possibly other values after this:
ft_id
123;1
garbled;2
special;3
The "2" and "3" would refer to other reference tables.
If this is the situation, then it would be cleaner to have a separate column with the correct ac_ste_trans_reference. However that occupies additional space, and can require multiple additional columns for each type. So hacks like the one you see are sometimes implemented.
Yes it is used for concatenation.
But only somebody having worked on this database model can explain what table data represent and why this concatenation is needed for this joining condition.
Right now i have 100 tables in SQL and i am looking for a specific string value in all tables, and i do not know which column it is in.
select * from table1, table2 where column1 = 'MyLostString' will not work because i do not know which column it has to be in.
Is there a SQL query for that, must i brute force search every table for every column for that 'MyLostString'
If I were to brute-force search across all tables, is there an efficient query for that?
For instance:
select * from table3 where allcolumns = MyLostString
It is the defining feature of a RDBMS (or at least one of them), that the meaning of a value depends on the column it is in. E.g.: The value 17 will have quite different meanings, if it stands in a customer_id column, than in the product_id of a fictional orders table.
This leads to the fact, that RDBMS are not well equipped to search for a value, no matter in which column of which tables it might be used.
My recommendation is to first study the data model to try and find out, which column of which table should be holding the value. If this really fails, you have a problem much worse than a "lost string".
The last ressort is to transform the DB into something better suited for fulltext search ... such as a flat file. You might want to try mydbexportcommand --options | grep -C10 'My lost string' or friends.
I have a VB app that accesses a sql database. I think it’s running slow, and I thought maybe I didn’t have the tables propery indexed. I was wondering how you would create the indexes? Here’s the situation.
My main loop is
Select * from Docrec
Order by YearFiled,DocNumb
Inside this loop I have two others databases hits.
Select * from Names
Where YearFiled = DocRec.YearFiled
and Volume = DocRec.Volume and Page = DocRec.Page
Order by SeqNumb
Select * from MapRec
Where FiledYear = DocRec.YearFiled
and Volume = DocRec.Volume and Page = DocRec.Page
Order by SeqNumb
Hopefully I made sense.
Try in one query using INNER JOIN:
SELECT * FROM Doctec d
INNER JOIN Names n ON d.YearField = n.YearField AND d.Volume = n.Volume AND d.Page = n.Page
INNER JOIN MapRec m ON m.FiledYear = n.YearFiled AND m.Volume = n.Volumen and m.Page = n.Page
ORDER BY YearFiled, DocNumb
You will have only one query to database. The problem can be that you hit database many times and get only one (or few) row(s) per time.
Off the top, one thing that would help would be determining if you really need all columns.
If you don't, instead of SELECT *, select just the columns you need - that way you're not pulling as much data.
If you do, then from SQL Server Management Studio (or whatever you use to manage the SQL Server) you'll need to look at what is indexed and what isn't. The columns you tend to search on the most would be your first candidates for an index.
Addendum
Now that I've seen your edit, it may help to look at why you're doing the queries the way you are, and see if there isn't a way to consolidate it down to one query. Without more context I'd just be guessing at more optimal queries.
In general looping through records is a poor idea. can you not do a set-based query that gives you everything you need in one pass?
As far as indexing consider any fields that you use in the ordering or where clauses and any fileds that arein joins. Primary keys are indexed as part of the setup of a primary ley but foreign keys are not. Often people forget that they need to index them as well.
Never use select * in a production environment. It is a poor practice. Do not ever return more data than you need.
I don't know if you need the loop. If all you are doing is grabbing the records in maprec that match for docrec and then the same for the second table then you can do this without a loop using inner join syntax.
select columnlist from maprec m inner join docrec d on (m.filedyear = d.yearfield and m.volume = d.volume and m.page=d.page)
and then again for the second table...
You could also trim up your queries to return only the columns needed instead of returning all if possible. This should help performance.
To create an index by yourself in SQL Server 2005, go to the design of the table and select the Manage Indexes & Keys toolbar item.
You can use the Database Engine Tuning Advisor. You can create a trace (using sql server profiler) of your queries and then the Advisor will tell you and create the indexes needed to optimize for your query executions.
UPDATE SINCE YOUR FIRST COMMENT TO ME:
You can still do this by running the first query then the second and third without a loop as I have shown above. Here's the trick. I am thinking you need to tie the first to the second and third one hence why you did a loop.
It's been a while since I have done VB6 recordsets BUT I do recall the ability to filter the recordset once returned from the DB. So, in this case, you could keep your loop but instead of calling SQL every time in the loop you would simply filter the resulting recordset data based on the first record. You would initialize / load the second & third query before this loop to load the data. Using the syntax above that I gave will load in each of those tables the matching to the parent table (docrec).
With this, you will still only hit the DB three times but still retain the loop you need to have the parent docrec table traversed so you can do work on it AND the child tables when you do have a match.
Here's a few links on ado recordset filtering....
http://www.devguru.com/technologies/ado/QuickRef/recordset_filter.html
http://msdn.microsoft.com/en-us/library/ee275540(BTS.10).aspx
http://www.w3schools.com/ado/prop_rs_filter.asp
With all this said.... I have this strange feeling that perhaps it could be solved with just a left join on your tables?
select * from docrec d
left join maprec m on (d.YearFiled= m.FiledYear and d.Volume = m.Volume and d.Page = m.Page)
left join names n on (d.YearFiled = n.YearFiled and d.Volume = n.Volume and d.Page = n.Page)
this will return all DocRec records AND add all the maprec values and name values where it matches OR NULL if not.
If this fits your need it will only hit the DB once.
Ok, here is my dilemma I have a database set up with about 5 tables all with the exact same data structure. The data is separated in this manner for localization purposes and to split up a total of about 4.5 million records.
A majority of the time only one table is needed and all is well. However, sometimes data is needed from 2 or more of the tables and it needs to be sorted by a user defined column. This is where I am having problems.
data columns:
id, band_name, song_name, album_name, genre
MySQL statment:
SELECT * from us_music, de_music where `genre` = 'punk'
MySQL spits out this error:
#1052 - Column 'genre' in where clause is ambiguous
Obviously, I am doing this wrong. Anyone care to shed some light on this for me?
I think you're looking for the UNION clause, a la
(SELECT * from us_music where `genre` = 'punk')
UNION
(SELECT * from de_music where `genre` = 'punk')
It sounds like you'd be happer with a single table. The five having the same schema, and sometimes needing to be presented as if they came from one table point to putting it all in one table.
Add a new column which can be used to distinguish among the five languages (I'm assuming it's language that is different among the tables since you said it was for localization). Don't worry about having 4.5 million records. Any real database can handle that size no problem. Add the correct indexes, and you'll have no trouble dealing with them as a single table.
Any of the above answers are valid, or an alternative way is to expand the table name to include the database name as well - eg:
SELECT * from us_music, de_music where `us_music.genre` = 'punk' AND `de_music.genre` = 'punk'
The column is ambiguous because it appears in both tables you would need to specify the where (or sort) field fully such as us_music.genre or de_music.genre but you'd usually specify two tables if you were then going to join them together in some fashion. The structure your dealing with is occasionally referred to as a partitioned table although it's usually done to separate the dataset into distinct files as well rather than to just split the dataset arbitrarily. If you're in charge of the database structure and there's no good reason to partition the data then I'd build one big table with an extra "origin" field that contains a country code but you're probably doing it for legitimate performance reason.
Either use a union to join the tables you're interested in http://dev.mysql.com/doc/refman/5.0/en/union.html or by using the Merge database engine http://dev.mysql.com/doc/refman/5.1/en/merge-storage-engine.html.
Your original attempt to span both tables creates an implicit JOIN. This is frowned upon by most experienced SQL programmers because it separates the tables to be combined with the condition of how.
The UNION is a good solution for the tables as they are, but there should be no reason they can't be put into the one table with decent indexing. I've seen adding the correct index to a large table increase query speed by three orders of magnitude.
The union statement cause a deal time in huge data. It is good to perform the select in 2 steps:
select the id
then select the main table with it
A cross join performs a cartesian product on the tuples of the two sets.
SELECT *
FROM Table1
CROSS JOIN Table2
Which circumstances render such an SQL operation particularly useful?
If you have a "grid" that you want to populate completely, like size and color information for a particular article of clothing:
select
size,
color
from
sizes CROSS JOIN colors
Maybe you want a table that contains a row for every minute in the day, and you want to use it to verify that a procedure has executed each minute, so you might cross three tables:
select
hour,
minute
from
hours CROSS JOIN minutes
Or you have a set of standard report specs that you want to apply to every month in the year:
select
specId,
month
from
reports CROSS JOIN months
The problem with maintaining these as views is that in most cases, you don't want a complete product, particularly with respect to clothes. You can add MINUS logic to the query to remove certain combinations that you don't carry, but you might find it easier to populate a table some other way and not use a Cartesian product.
Also, you might end up trying the cross join on tables that have perhaps a few more rows than you thought, or perhaps your WHERE clause was partially or completely missing. In that case, your DBA will notify you promptly of the omission. Usually he or she will not be happy.
Generate data for testing.
You're typically not going to want a full Cartesian product for most database queries. The whole power of relational databases is that you can apply whatever restrictions you might be interested in to allow you to avoid pulling unnecessary rows from the db.
I suppose one contrived example where you might want that is if you have a table of employees and a table of jobs that need doing and want to see all possible assignments of one employee to one job.
The key is "show me all possible combinations". I've used these in conjunction with other calculated fields an then sorted/filtered those.
For example, say you are building an arbitrage (trading) application. You have sellers offering products at a price and buyers asking for products at a cost. You do a cross join on the product key (to match up the potential buyers and sellers), calculate the spread between cost and price, then sort desc. on this to give you (the middleman) the most profitable trades to execute. Almost always you'll have other bounding filter criteria of course.
Ok, this probably won't answer the question, but, if it's true (and I'm not even sure of that) it's a fun bit of history.
In the early days of Oracle, one of the developers realized that he needed to duplicate every row in a table (for example, it's possible it was a table of events and he needed to change it separate "start event" and "end event" entries). He realized that if he had a table with just two rows, he could do a cross join, selecting just the columns in the first table, and get exactly had he needed. So he created a simple table, which he naturally enough called "DUAL".
Later, he need to do something which could only be done via a select from a table, even though the action itself had nothing to do with the table, (perhaps he forgot his watch and wanted to read the time via SELECT SYSDATE FROM...) He realized that he still had his DUAL table lying around, and used that. After a while, he tired of seeing the time printed twice, so he eventual deleted one of the rows.
Others at Oracle started using his table, and eventually, it was decided to include it in the standard Oracle installation.
Which explains why a table whose only significance is that it has one row has a name which means "two".
Takes something like a digits table, which has ten rows for the digits 0-9. You can use cross join on that table a few times to a get a result with however many rows you need, and each row will be numbered appropriately. This has a number of uses. For example, you can combine it with a dateadd() function to get a set for every day in a given year.
Note: this post is old now. Today I'd use generate_series() or a recursive CTE to do this job instead.
This is an interesting way to use a cross join to create a crosstab report. I found it in Joe Celko's SQL For Smarties, and have used it several times. It does take a little setup, but has been worth the time invested.
you can use it CROSS JOIN to:
generate data for testing purposes
combine all properties - you need all possible combination of e.g blood groups (A,B,..) with Rh-/+, etc...
--tune it for your purposes;) - I'm not expert in this area;)
CREATE TABLE BL_GRP_01 (GR_1 text);
CREATE TABLE RH_VAL_01 (RH_VAL text);
INSERT INTO BL_GRP_01 VALUES ('A'), ('B'), ('AB'), ('O');
INSERT INTO RH_VAL_01 VALUES ('+'), ('-');
SELECT CONCAT(x.GR_1, y.RH_val)
FROM BL_GRP_01 x
CROSS JOIN RH_VAL_01 y
ORDER BY CONCAT(x.GR_1, y.RH_VAL);
create a join for 2 tables without a common id and then group it using max(),etc.. to find highest possible combination
Imagine you had a series of queries you want to issue over a specific combination of items and dates (prices, availability, etc..). You could load the items and dates into separate temp tables and have your queries cross join the tables. This may be more convenient than the alternative of enumerating the items and dates in IN clauses, especially since some databases limit the number of elements in an IN clause.