Stupidly simple question, but I just don't know what to google!
If I create a query like this:
Select id, data
from table1
Now I want to join with table2. I can immediately see that the id column is no longer unique and I have to change it to
table1.id
Is there any smart way (like a keyboard-shortcut) to do this, instead of manually adding table1 to every column? Either before I add the Join to secure that all columns will be unique, or after with suggestions based on the different possible tables.
No, there is no helper.
But do not you can alias the table name:
select x.Col1, y.Col2
from ALongTableName x
inner join AReallyReallyLongTableName y on x.Id = y.OtherId
which can also make queries clearer, and is very much necessary when doing self joins.
First of all, you should start using aliases:
SQL aliases are used to give a database table, or a column in a table,
a temporary name.
Basically aliases are created to make column names more readable.
This will narrow down your problem and make your code maintenance easier. If that's not enough, I guess you could start using auto-completion tools, such as these:
SQL Complete
SQL Prompt
ApexSQL Complete
These have your desired functionality, however, they do not always work as expected (at least for me).
Oh! You can use alias table name. Like this:
SELECT A.ID, A.data
FROM TableA A
INNER JOIN TableB B
ON A.ID = B.ID
You just only use A. or B. if two table have same this column selected. If they different, you don't need: Like this:
SELECT A.ID, data -- if Table B not have column data
FROM TableA A
INNER JOIN TableB B
ON A.ID = B.ID
Or:
Select A.*, B.ID
FROM TableA A
INNER JOIN TableB B
ON A.ID = B.ID
Related
I have two tables
Table A
(
id int,
name varchar(39),
lname varchar (49),
...
)
Table B
(
id int,
city varchar(39),
...
)
Both tables are sorted on column ID. IDs are simply identities and are populated by auto incremented integers 1 to n.
However, if I input a query e.g.,
SELECT *
FROM A, B
WHERE A.id = B.id;
I get a hash join instead of the efficient merge join. How can I enforce the merge join in SQL Server instead? I don't want to use an index, thus no index-based plans.
Note that I don't want a merge-join with a sort-enforcer either, I know that one can hint the planner by rewriting the query to
SELECT *
FROM A
INNER MERGE JOIN B ON A.ID = B.ID;
By the way I'm using SQL Server Express edition. But I can change to any open source DB if the latter supports the query plan that I'm aiming.
Thanks in advance
If you believe you are smarted then the SQL Engine :-) you can use hits like this:
SELECT *
FROM A
INNER HASH JOIN B
ON A.id = B.id
OR
SELECT *
FROM A
INNER MERGE JOIN B
ON A.id = B.id
At least, you can test if the MERGE will be really better. And even it is better in this case, does not mean that it will be the best choice always. It can reduce the performance in other cases, so generally it will be better to leave this work to the engine.
So i have to do a cartesian product (or CROSS JOIN) between two tables. One problem is that both tables have a column with the name 'itemname'. My current case looks as follows:
select *
into #cartesian_temp
from xsale CROSS JOIN xitem
delete from #cartesian_temp where deptname='books' and itemcolor='bamboo'
select * from #cartesian_temp
so the error I get is:
Column names in each table must be unique. Column name 'itemname' in table '#cartesian_temp' is specified more than once
Anyone that can help me with my problem?
you can add alias for the columns like below.
select XS.itemname as saleitemname , XI.itemname as saleitemname2
into #cartesian_temp
from xsale XS CROSS JOIN xitem XI
This is one of the reasons why seasoned SQL pro's will ALWAYS advocate to give Tables an alias and to ALWAYS fully qualify every column name by using the alias. It's not just a cross join problem
Avoid this:
SELECT *
FROM
person
INNER JOIN
address
ON addressid = address.id -- Person.addressid
Sure, it'll work as long as the column names are all unique (it'll probably cause issues even now because person will have an I'd column and so will address) but it might stop working at any point in future if someone adds columns to either table with names that clash
Prefer this:
SELECT p.id as personid, a.id as addressid, p.name, a.zipcode
FROM
person p
INNER JOIN
address a
ON p.addressid = a.id
This is fully aliased (both tables have an alias) and we haven't used select *; weve fully qualified every column with a prefix using the table alias and we've aliased columns that have the same named (the ID columns) in each table so we can tel them apart. No one can add any columns to the db and cause this query to stop working
Aliasing Tables helps in another way; it lets us use the same table twice in a query. Suppose a person had a work address and a home address:
SELECT ...
FROM
person p
INNER JOIN
address awork
ON p.workaddressid = awork.id
INNER JOIN
address ahome
ON p.homeaddressid = ahome.id
This is impossible without aliasing. Always give aliases a sensible name (not a1, a2)
For your case, go like:
SELECT xs.itemname as xsitemname, xi.itemname as xiitemname, ...
FROM
xsale xs
CROSS JOIN
xitem xi
WHERE
xi.itemcolor = 'green'
This will be every green item crossed with every sale item
We have a query that looks for discrepancies between two tables in two different databases (one SQL Server, one Oracle) that in theory should always be in sync. The query pulls the data from both tables into table variables and then does a FULL OUTER JOIN to find the discrepancies. We suspect that the FULL OUT JOIN is partially to blame for the performance issues.
Would it make sense to rely on two LEFT OUTER JOINs and look for records that don't exist on the right side of the join?
We're also thinking about using temp tables to further help with performance.
One option is to do a inner join and store results in a temp table. Then do a select from TableA where not exists in tempTableWithCommonRecords
and another select from TableB where not exists in tempTableWithCommonRecords
Cant say if this will perform better as don't have enough info for that. Its just another option.
You can try the EXCEPT operator which handles complex joins, and it should work in both PL-SQL and T-SQL. It will return any values in the left table that are not a perfect match on the right table:
SELECT [Field1], [Field2], [Field3]
FROM Table1
EXCEPT
SELECT [Field1], [Field2], [Field3]
FROM Table2
UNION
SELECT [Field1], [Field2], [Field3]
FROM Table2
EXCEPT
SELECT [Field1], [Field2], [Field3]
FROM Table1
I am trying to build hierarchies by nesting queries on the same table in MS SQL Server 2014.
To give an example of what I am trying to achieve:
I have a table 'employees' whith the following columns:
[ID],[First Name],[Last Name],[ReportsTo]
{1},{John},{Doe},{2}
{2},{Mary},{Miller},{NULL}
I am trying to build a statement, where I join the employees table with itself and where I build a hierarchy with the boss on top.
Expected Result:
[Employee],[Boss]
{Miller,Mary},{NULL}
{Doe, John},{Miller,Mary}
I apologize, if this is a stupid question, but I fail to create a working nested query.
Could you please help me with that?
Thank you very much in advance
Based on the intended results, it looks like what you essentially want is a list of employees. So let's start with that:
SELECT LastName, FirstName, ReportsTo FROM Employees
This gives you the list, so you now have the objects you're looking for. But you need to fill out more data. You want to follow ReportsTo and show data from the record to which that points as well. This would be done exactly as it would if the foreign key pointed to a different table. (The only difference from being the same table is that you must use table aliases in the query, since you're including the same table twice.)
So let's start by joining the table:
SELECT e.LastName, e.FirstName, e.ReportsTo
FROM Employees e
LEFT OUTER JOIN Employees b on e.ReportsTo = b.ID
The results should still be the same, but now you have more data to select from. So you can add the new columns to the SELECT clause:
SELECT
e.LastName AS EmployeeLastName,
e.FirstName AS EmployeeFirstName,
b.LastName AS BossLastName,
b.FirstName AS BossFirstName
FROM Employees e
LEFT OUTER JOIN Employees b on e.ReportsTo = b.ID
It's a join like any other, it just happens to be a join to the same table.
If you add more than one table to the FROM clause (in a query), how does this impact the result set? Does it first select from the first table then from the second and then create a union (i.e., only the rowspace is impacted?) or does it actually do something like a join (i.e., extend the column space)? And when you use multiple tables in the FROM clause, does the WHERE clause filter both sub-result-sets?
Specifying two tables in your FROM clause will execute a JOIN. You can then use the WHERE clause to specify your JOIN conditions. If you fail to do this, you will end-up with a Cartesian product (every row in the first table indiscriminately joined to every row in the second).
The code will look something like this:
SELECT a.*, b.*
FROM table1 a, table2 b
WHERE a.id = b.id
However, I always try to explicitly specify my JOINs (with JOIN and ON keywords). That makes it abundantly clear (for the next developer) as to what you're trying to do. Here's the same JOIN, but explicitly specified:
SELECT a.*, b.*
FROM table1 a
INNER JOIN table2 b ON b.id = a.id
Note that now I don't need a WHERE clause. This method also helps you avoid generating an inadvertent Cartesian product (if you happen to forget your WHERE clause), because the ON is specified explicitly.