I'm trying to achieve 2 joins. If I run the 1st join alone it pulls 4 lots of results, which is correct. However when I add the 2nd join which queries the same reference table using the results from the select statement it pulls in additional results. Please see attached. The squared section should not be being returned
So I removed the 2nd join to try and explain better. See pic2. I'm trying to get another column which looks up InvolvedInternalID against the initial reference table IRIS.Practice.idvClient.
Your database is simply doing as you tell it. When you add in the second join (confusingly aliased as tb1 in a 3 table query) the database is finding matching rows that obey the predicate/truth statement in the ON part of the join
If you don't want those rows in there then one of two things must be the case:
1) The truth you specified in the ON clause is faulty; for example saying SELECT * FROM person INNER JOIN shoes ON person.age = shoes.size is faulty - two people with age 13 and two shoes with size 13 will produce 4 results, and shoe size has nothing to do with age anyway
2) There were rows in the table joined in that didn't apply to the results you were looking for, but you forgot to filter them out by putting some WHERE (or additional restriction in the ON) clause. Example, a table holds all historical data as well as current, and the current record is the one with a NULL in the DeletedOn column. If you forget to say WHERE deletedon IS NULL then your data will multiply as all the past rows that don't apply to your query are brought in
Don't alias tables with tbX, tbY etc.. Make the names meaningful! Not only do aliases like tbX have no relation to the original table name (so you encounter tbX, and then have to go searching the rest of the query to find where it's declared so you can say "ah, it's the addresses table") but in this case you join idvclient in twice, but give them unhelpful aliases like tb1, tb3 when really you should have aliased them with something that describes the relationship between them and the rest of the query tables
For example, ParentClient and SubClient or OriginatingClient/HandlingClient would be better names, if these tables are in some relationship with each other.
Whatever the purpose of joining this table in twice is, alias it in relation to the purpose. It may make what you've done wriong easier to spot, for example "oh, of course.. i'm missing a WHERE parentclient.type = 'parent'" (or WHERE handlingclient.handlingdate is not null etc..)
The first step to wisdom is by calling things their proper names
Suppose you have a department table with DepartmentID as primary key, and an employee table with DepartmentID as a foreign key. You can then use the fact that these columns have the same name, to perform a natural join that allows you to omit the column name from the query. (I'm not commenting on whether you should or not - that's a matter of opinion - just noting the fact that this shorthand is part of SQL syntax.)
There are various other cases in SQL syntax where you might refer to the column names with expressions like employee.DepartmentID = department.DepartmentID. Are there any other cases where some kind of shorthand allows you to use the fact that the columns have the same name, to omit the column name?
SQL does not know directly about foreign keys; it just has foreign key constraints, which prevent you from creating invalid data. When you have a foreign key, you would want both a constraint and to do joins on it, but the database does not automatically derive one from the other.
Anyway, when you are using a join on two columns with the same names:
SELECT ...
FROM employee
JOIN department ON employee.DepartmentID = department.DepartmentID
then you can replace the ON clause with the USING clause:
SELECT ...
FROM employee
JOIN department USING (DepartmentID)
If there is a USING clause then each of the column names specified must exist in the datasets to both the left and right of the join-operator. For each pair of named columns, the expression "lhs.X = rhs.X" is evaluated for each row of the cartesian product as a boolean expression. Only rows for which all such expressions evaluates to true are included from the result set.
[…]
For each pair of columns identified by a USING clause, the column from the right-hand dataset is omitted from the joined dataset. This is the only difference between a USING clause and its equivalent ON constraint.
(Omitting the duplicate column matters only when you are using SELECT *. (I'm not commenting on whether you should or not – that's a matter of opinion – just noting the fact that this shorthand is part of SQL syntax.))
I'm a college student and the database I'm working with is purely fictional but part of it requires me to make a query that is a join.
What I have are 3 tables each with part of the data but also needing to use two of those tables as conditions based off the main table. What I mean is I have an employee table, order table and customer table and the only thing any two of them have in common is the ID of either the employee or the customer is part of the order table.
Now what I am trying to do is create a join statement that will get certain information from the employee and customer tables and only those that both the employee and the customer are also on the same line in the order table. How should i make this type of conditional statement?
Any example using the same basic scenario will work I can use that to help me build my own query.
This is what I have right now:
SELECT [Customer/Vendor_Info_local].Name_of_customer,
Employee_Info_local.Employee_Name
FROM Employee_Info_local,
[Customer/Vendor_Info_local],
Order_Information_local
WHERE (([Customer/Vendor_Info_local].[Customer/VendorID] =
[Order_Information_local].[Cusrtomer/VendorID])
AND
([Employee_Info_local].[EmployeeID] = [Order_Information_local].[EmployeeID]));
I keep getting a type mismatch error when i try to use it and honestly not even sure what that means.
I noticed when doing a query with multiple JOINs that my query didn't work unless I gave one of the table names an alias.
Here's a simple example to explain the point:
This doesn't work:
SELECT subject
from items
join purchases on items.folder_id=purchases.item_id
join purchases on items.date=purchases.purchase_date
group by folder_id
This does:
SELECT subject
from items
join purchases on items.folder_id=purchases.item_id
join purchases as p on items.date=p.purchase_date
group by folder_id
Can someone explain this?
You are using the same table Purchases twice in the query. You need to differentiate them by giving a different name.
You need to give an alias:
When the same table name is referenced multiple times
Imagine two people having the exact same John Doe. If you call John, both will respond to your call. You can't give the same name to two people and assume that they will know who you are calling. Similarly, when you give the same resultset named exactly the same, SQL cannot identify which one to take values from. You need to give different names to distinguish the result sets so SQL engine doesn't get confused.
Script 1: t1 and t2 are the alias names here
SELECT t1.col2
FROM table1 t1
INNER JOIN table1 t2
ON t1.col1 = t2.col1
When there is a derived table/sub query output
If a person doesn't have a name, you call them and since you can't call that person, they won't respond to you. Similarly, when you generate a derived table output or sub query output, it is something unknown to the SQL engine and it won't what to call. So, you need to give a name to the derived output so that SQL engine can appropriately deal with that derived output.
Script 2: t1 is the alias name here.
SELECT col1
FROM
(
SELECT col1
FROM table1
) t1
The only time it is REQUIRED to provide an alias is when you reference the table multiple times and when you have derived outputs (sub-queries acting as tables) (thanks for catching that out Siva). This is so that you can get rid of ambiguities between which table reference to use in the rest of your query.
To elaborate further, in your example:
SELECT subject
from items
join purchases on items.folder_id=purchases.item_id
join purchases on items.date=purchases.purchase_date
group by folder_id
My assumption is that you feel that each join and its corresponding on will use the correlating table, however you can use whichever table reference you want. So, what happens is that when you say on items.date=purchases.purchase_date, the SQL engine gets confused as to whether you mean the first purchases table, or the second one.
By adding the alias, you now get rid of the ambiguities by being more explicit. The SQL engine can now say with 100% certainty which version of purchases that you want to use. If it has to guess between two equal choices, then it will always throw an error asking for you to be more explicit.
It is required to give them a name when the same table is used twice in a query. In your case, the query wouldn't know what table to choose purchases.purchase_date from.
In this case it's simply that you've specified purchases twice and the SQL engine needs to be able to refer to each dataset in the join in a unique way, hence the alias is needed.
As a side point, do you really need to join into purchases twice? Would this not work:
SELECT
subject
from
items
join purchases
on items.folder_id=purchases.item_id
and items.date=purchases.purchase_date
group by folder_id
The alias are necessary to disambiguate the table from which to get a column.
So, if the column's name is unique in the list of all possible columns available in the tables in the from list, then you can use the coulmn name directly.
If the column's name is repeated in several of the tables available in the from list, then the DB server has no way to guess which is the right table to get the column.
In your sample query all the columns names are duplicated because you're getting "two instances" of the same table (purchases), so the server needs to know from which of the instance to take the column. SO you must specify it.
In fact, I'd recommend you to always use an alias, unless there's a single table. This way you'll avoid lots of problems, and make the query much more clear to understand.
You can't use the same table name in the same query UNLESS it is aliased as something else to prevent an ambiguous join condition. That's why its not allowed. I should note, it's also better to use always qualify table.field or alias.field so other developers behind you don't have to guess which columns are coming from which tables.
When writing a query, YOU know what you are working with, but how about the person behind you in development. If someone is not used to what columns come from what table, it can be ambiguous to follow, especially out here at S/O. By always qualifying by using the table reference and field, or alias reference and field, its much easier to follow.
select
SomeField,
AnotherField
from
OneOfMyTables
Join SecondTable
on SomeID = SecondID
compare that to
select
T1.SomeField,
T2.AnotherField
from
OneOfMyTables T1
JOIN SecondTable T2
on T1.SomeID = T2.SecondID
In these two scenarios, which would you prefer reading... Notice, I've simplified the query using shorter aliases "T1" and "T2", but they could be anything, even an acronym or abbreviated alias of the table names... "oomt" (one of my tables) and "st" (second table). Or, as something super long as has been in other posts...
Select * from ContractPurchaseOffice_AgencyLookupTable
vs
Select * from ContractPurchaseOffice_AgencyLookupTable AgencyLkup
If you had to keep qualifying joins, or field columns, which would you prefer looking at.
Hope this clarifies your question.
I'm not entirely sure if there's a standard in the industry or otherwise, so I'm asking here.
I'm naming a Users table, and I'm not entirely sure about how to name the members.
user_id is an obvious one, but I wonder if I should prefix all other fields with "user_" or not.
user_name
user_age
or just name and age, etc...
prefixes like that are pointless, unless you have something a little more arbitrary; like two addresses. Then you might use address_1, address_2, address_home, etc
Same with phone numbers.
But for something as static as age, gender, username, etc; I would just leave them like that.
Just to show you
If you WERE to prefix all of those fields, your queries might look like this
SELECT users.user_id FROM users WHERE users.user_name = "Jim"
When it could easily be
SELECT id FROM users WHERE username = "Jim"
I agree with the other answers that suggest against prefixing the attributes with your table names.
However, I support the idea of using matching names for the foreign keys and the primary key they reference1, and to do this you'd normally have to prefix the id attributes in the dependent table.
Something which is not very well known is that SQL supports a concise joining syntax using the USING keyword:
CREATE TABLE users (user_id int, first_name varchar(50), last_name varchar(50));
CREATE TABLE sales (sale_id int, purchase_date datetime, user_id int);
Then the following query:
SELECT s.*, u.last_name FROM sales s JOIN users u USING (user_id);
is equivalent to the more verbose and popular joining syntax:
SELECT s.*, u.last_name FROM sales s JOIN users u ON (u.user_id = s.user_id);
1 This is not always possible. A typical example is a user_id field in a users table, and reported_by and assigned_to fields in the referencing table that both reference the users table. Using a user_id field in such situations is both ambiguous, and not possible for one of the fields.
As other answers suggest, it is a personal preference - pick up certain naming schema and stick to it.
Some 10 years ago I worked with Oracle Designer and it uses naming schema that I like and use since then:
table names are plural - USERS
surrogate primary key is named as singular of table name plus '_id' - primary key for table USERS would be "USER_ID". This way you have consistent naming when you use "USER_ID" field as foreign key in some other table
column names don't have table name as prefix.
Optionally:
in databases with large number of tables (interpret "large" as you see fit), use 2-3
characters table prefixes so that you can logically divide tables in areas. For example: all tables that contain sales data (invoices, invoice items, articles) have prefix "INV_", all tables that contain human resources data have prefix "HR_". That way it is easier to find and sort tables that contain related data (this could also be done by placing tables in different schemes and setting appropriate access rights, but it gets complicated when you need to create more than one database on one server)
Again, pick naming schema you like and be consistent.
Just go with name and age, the table should provide the necessary context when you're wondering what kind of name you're working with.
Look at it as an entity and name the fields accordingly
I'd suggest a User table, with fields such as id, name, age, etc.
A group of records is a bunch of users, but the group of fields represents a user.
Thus, you end up referring to user.id, user.name, user.age (though you won't always include the table name, depending on the query).
For the table names, I usually use pluralized nouns (or noun phrases), like you.
For column names I'd not use the table name as prefix. The table itself specifies the context of the column.
table users (plural):
id
name
age
plain and simple.
It's personal preference. The best advice we can give you is consistency, legibility and ensuring the relationships are correctly named as well.
Use names that make sense and aren't abbreviated if possible, unless the storage mechanism you are using doesn't work well with them.
In relationships, I like to use Id on the primary key and [table_name]_Id on the foreign key. eg. Order.Id and OrderItem.OrderId
Id works well if using a surrogate key as a primary key.
Also your storage mechanism may or may not be case sensitive, so be sure to that into account.
Edit: Also, thre is some theory to suggest that table should be name after what a single record in that table should represent. So, table name "User" instead of "Users" - personally the plural makes more sense to me, just keep it consistent.
First of all, I would suggest using the singular noun, i.e. user instead of users, although this is more of a personal preference.
Second, there are some who prefer to always name the primary key column id, instead of user_id (i.e. table name + id), and similar with for example name instead of employee_name. I think this is a bad idea for the following reason:
-- when every table has an "id" (or "name") column, you get duplicate column names in the output:
select e.id, e.name, d.id, d.name
from employee e, department d
where e.department_id = d.id
-- to avoid this, you need to specify column aliases every time you query:
select e.id employee_id, e.name employee_name, d.id department_id, d.name department_name
from employee e, department d
where e.department_id = d.id
-- if the column name includes the table, there are no conflicts, and the join condition is very clear
select e.employee_id, e.employee_name, d.department_id, d.department_name
from employee e, department d
where e.department_id = d.department_id
I'm not saying you should include the table name in every column in the table, but do it for the key (id) column and other "generic" columns such as name, description, remarks, etc. that are likely to be included in queries.
I explicitly named my columns using a prefix that was related to the table
i.e. table = USERS, column name = user_id, user_name, user_address_street, etc.
before that, when i started using JOINS I had to alias the crap out of the column names to avoid conflict in the query results, and then when accessed from templates in a MVC View, if the query result field name didn't match the published db schema, the template designers would get all confused and have to ask for the SQL VIEW to determine the correct field name to use.
So it looks messy to use a prefix in a column name, but in practice it works better for us.
I'm not entirely sure if there's a standard in the industry
Yes: ISO 11179-5: Naming and identification principles, available here.
I think table and column names must be like that.
Table Name :
User --> Capitalize Each Word and not plural.
Column Names :
Id --> If i see "Id" I understand this is PK column.
GroupId --> I understanding there is an table which named Group and this column is relation column for Group table.
Name --> If there is a column which named "Name" in User table, this means name of user. It's enaughly clear.
Especially if you are using Entity Framework I suppose this more.
Note: Sorry for my bad English. If somebody will correct my bad English i will be happy.