如何从两个表中进行选择并在SQL中匹配两列？

Question

So what i'm trying to do is to SELECT all records from table1 that has a certain Project number. Then i will JOIN another table and get only one column named "FloorId" that is equal to both Project AND Element in table1.

(SELECT Project FROM dbo.IMP_MODEL_GEOMETRY WHERE dbo.IMP_GEOMETRY.Project = dbo.IMP_ELEMENT.Project AND dbo.IMP_GEOMETRY.Element = dbo.IMP_ELEMENT.Element)

In the dbo.IMP_GEOMETRY table i will get several results on Project and Element, so maby i can just select the first one.. DISTINCT?

This is what i have tried:

$sql = "SELECT * FROM dbo.IMP_ELEMENT WHERE Project LIKE '%$objNr%'
            INNER JOIN dbo.IMP_MODEL_GEOMETRY ON dbo.IMP_ELEMENT.Project = dbo.IMP_MODEL_GEOMETRY.Project";

I am new to SQL and don't really know what the "." stands for in "dbo.IMP". The table name is: dbo.IMP_ELEMENT (I have seen querys where the dot marks the table and after the dot is the column name. But in this case the dot doesn't represent both table and column, it's just a dot in the table name.

Except in the JOIN where Project is the column (dbo.IMP_MODEL_GEOMETRY.Project)

The table i expect looks something like:

id    Project    Element    FloorId

Where FloorId comes from table2.

EDIT

Both tables contains "Project" and "Element" So, i can use them to match. In table1 there is only one row per Project-Element but in table2 there can be multiple rows with Project-Element, so from table2 it's ok to only select the first match found.

Answer 1

You can try below way -

SELECT a.id,a.project,a.element,b.floorid FROM dbo.IMP_ELEMENT a
            INNER JOIN dbo.IMP_MODEL_GEOMETRY b ON a.Project = b.Project and a.element=b.element
WHERE a.Project LIKE '%$objNr%'

Answer 2

I am new to SQL and don't really know what the "." stands for in "dbo.IMP".

The "." is part of the syntax to identify the element you are after. In full this is .. eg Adventureworks.dbo.customer

This extends to columns as well by adding another "." and specifying the column name.

But developers are lazy, and if you do not specify the database then the engine uses the database the query is currently connected to. It is totally normal to not specify a database.

Actually we are really lazy and most don't specify the schema (the dbo. part) as the database engine will just check all schema.

Selecting data - joins

SELECT
    --use table aliases (t1.) for each column, this is shorthand for dbo.table1 
    --if the same column exists in more than one table the database engine knows 
    --which one you want by looking at the aliases  
    t1.column_name 
    ,t2.column_name
FROM 
    dbo.table1 as t1
    INNER JOIN dbo.table2 as t2  
        --inner join returns only rows where the join matches both tables 
        ON t1.primary_key_column = t2.foreign_key_column 
WHERE
    --the join gives you a super table with all the rows that matched the join
    --now you can add in any filters to your results
    t1.some_column = 'foo'
;

What's the difference between INNER JOIN, LEFT JOIN, RIGHT JOIN and FULL JOIN?

Answer 3

Or you can try the below,

select table1.*, table2.FloorId
from IMP_ELEMENT as table1,
     IMP_MODEL_GEOMETRY as table2 
where table1.Project = table2.Project
  and table1.Project like '%Whatever%';

You can change the mail table as whatever you like.