SQL: Spalten aus Tabellen verknüpfen

Hallo,

schau mal hier vielleicht hilft Dir das weiter! Das hatten wir mal in Wirtschaftsinformatik gemacht. Leider weiss ich die ursprüngliche Quelle nicht mehr. Falls sich durch die Veröffentlichung irgendwer auf den Schlips getreten fühlen sollte bitte melden, dann entferne ich den Text sofort wieder!

Table JOINS (a must)
All of the queries up until this point have been useful with the exception of one major limitation - that is, you've been selecting from only one table at a time with your SELECT statement. It is time to introduce you to one of the most beneficial features of SQL & relational database systems - the "Join". To put it simply, the "Join" makes relational database systems "relational".
Joins allow you to link data from two or more tables together into a single query result - from one single SELECT statement.
A "Join" can be recognized in a SQL SELECT statement if it has more than one table after the FROM keyword.
For example:

SELECT "list-of-columns"
FROM table1,table2
WHERE "search-condition(s)"

Joins can be explained easier by demonstrating what would happen if you worked with one table only, and didn't have the ability to use "joins". This single table data-base is also sometimes referred to as a "flat table". Let's say you have a one-table database that is used to keep track of all of your customers and what they purchase from your store:

id first last address city state zip date item price

Everytime a new row is inserted into the table, all columns will be be updated, thus resulting in unnecessary "redundant data". For example, every time Wolfgang Schultz purchases something, the following rows will be inserted into the table:

id first last address city sta-te zip date item price
10982 Wolf-gang Schultz 300 N. 1st Ave Yu-ma AZ 85002 032299 snowboard 45.00
10982 Wolf-gang Schultz 300 N. 1st Ave Yu-ma AZ 85002 082899 snow sho-vel 35.00
10982 Wolf-gang Schultz 300 N. 1st Ave Yu-ma AZ 85002 091199 gloves 15.00
10982 Wolf-gang Schultz 300 N. 1st Ave Yu-ma AZ 85002 100999 lantern 35.00
10982 Wolf-gang Schultz 300 N. 1st Ave Yu-ma AZ 85002 022900 tent 85.00

An ideal database would have two tables:
1) One for keeping track of your customers
2) And the other to keep track of what they purchase:
"Customer_info" table:

customer_number firstname lastname address city state zip

"Purchases" table:

customer_number date item price

Now, whenever a purchase is made from a repeating customer, the 2nd table, "Pur-chases" only needs to be updated! We've just eliminated useless redundant data, that is, we've just normalized this database!
Notice how each of the tables have a common "cusomer_number" column. This col-umn, which contains the unique customer number will be used to JOIN the two ta-bles. Using the two new tables, let's say you would like to select the customer's name, and items they've purchased. Here is an example of a join statement to ac-complish this:

SELECT customer_info.firstname, customer_info.lastname, purchases.item
FROM customer_info, purchases
WHERE customer_info.customer_number = purchases.customer_number;

This particular "Join" is known as an "Inner Join" or "Equijoin". This is the most com-mon type of "Join" that you will see or use.
Notice that each of the colums are always preceeded with the table name and a pe-riod. This isn't always required, however, it IS good practice so that you wont confuse which colums go with what tables. It is required if the name column names are the same between the two tables. I recommend preceeding all of your columns with the table names when using joins.
Note: The syntax described above will work with most Database Systems -including the one with this tutorial. However, in the event that this doesn't work with yours, please check your specific database documentation.
Although the above will probably work, here is the ANSI SQL-92 syntax specification for an Inner Join using the preceding statement above that you might want to try:

SELECT customer_info.firstname, customer_info.lastname, purchases.item
FROM customer_info INNER JOIN purchases
ON customer_info.customer_number = purchases.customer_number;

Another example:

SELECT employee_info.employeeid, employee_info.lastname, em-ployee_sales.comission
FROM employee_info, employee_sales
WHERE employee_info.employeeid = employee_sales.employeeid;

This statement will select the employeeid, lastname (from the employee_info table), and the comission value (from the employee_sales table) for all of the rows where the employeeid in the employee_info table matches the employeeid in the em-ployee_sales table.

Hallo,

um nochmal kurz das Problem zusammenzufassen:

Du hast zwei Tabellen, und möchtest
1. _alle_ Spalten aus Tabelle 1, mit zusätzlich der Spalte "Zusatzwert" aus Tabelle 2, sofern vorhanden
2. das JOIN-Kriterium ist die Artikelnummer (eine Zeichenkette), wobei diese in Tabelle 2 jeweils noch um " 00" erweitert ist.

Beispiel:

Gegeben seien die Tabellen Table1 und Table2 (hier mal der Kürze halber ohne primary key, erlaubten NULL-Werten etc.)

create table Table1
(
ArtNr varchar (35),
OtherInfo varchar (35)
)

create table Table2
(
ArtNr varchar (35),
AddValue varchar (35)
)
Die von Dir gesuchte Query lautet dann:

select T1.*, T2.AddValue
from Table1 T1 left outer join Table2 T2
  on ( CONCAT(T1.ArtNr,' 00') = T2.ArtNr )

Obige Punkte werden mit folgenden Konstrukten gelöst:

1. ein Left Outer Join (die Syntax im Beispiel funktioniert mit MySQL, MS SQL Server und vielen anderen, Oracle hat -- zumindest bis Version 8i, neuere weiss ich nicht -- eine eigene, unlesbarere Syntax :-))

2. bei Join-Kriterium musst Du die "Verlängerung" ' 00' an die Artikelnummer aus Tabelle 1 anhängen, wenn Du mit der Artikelnummer aus Tabelle 2 vergleichst. Auch hier wieder gilt: Die Syntax CONCAT( str1, str2, ...) ist diejenige von MySQL, derartige Funktionen können von Datenbank zu Datenbank unterschiedliche Syntax haben.

Und zum selber ausprobieren noch ein paar Testdaten:

insert into Table1 values ( '12345', 'Artikel 1' );
insert into Table1 values ( '67890', 'Artikel 2' );
insert into Table2 values ( '12345 00', 'Zusatzwert 1' );
insert into Table2 values ( '67890', 'Zusatzwert ohne Artikel in Tab 1');

und siehe da, das Ergebnis lautet:

+-------+-----------+--------------+
| ArtNr | OtherInfo | AddValue     |
+-------+-----------+--------------+
| 12345 | Artikel 1 | Zusatzwert 1 |
| 67890 | Artikel 2 | NULL         |
+-------+-----------+--------------+

das entspricht zumindest meinem oben beschriebenen Verständnis der Problemstellung

Ciao
Michael