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The bridge is built - summary

Well done, @name! You have crossed the Bridge of Knowledge and can already link the Library's shelves with cross-references. Here is what you have gained:

What you have learned

Data is split across tables linked by a foreign key (
```
ksiazki.autor_id
```
→
```
autorzy.id
```
).
INNER JOIN ... ON ... returns only matching pairs of rows from both tables.
The ON clause defines the join condition - on which column the tables find each other.
Table aliases (
```
k
```
,
```
a
```
,
```
c
```
,
```
w
```
,
```
kat
```
) shorten column names and make them unambiguous.
We can join many shelves in one query by adding further
```
JOIN
```
clauses.
LEFT JOIN keeps all rows from the left table; a missing match gives NULL.
Combining
```
LEFT JOIN
```
with
```
WHERE ... IS NULL
```
finds rows without a pair (e.g. readers with no loans).
RIGHT JOIN is the mirror image of
```
LEFT JOIN
```
- it keeps rows from the right table.
A self-join joins a table with itself, giving it two different aliases.

INNER versus LEFT - remember

| | INNER JOIN | LEFT JOIN | |---|------------|-----------| | Rows without a pair | disappear | stay (with NULL) | | When to use | we want only complete pairs | we want to keep all rows from the left |

What's next?

In the next location we will rise above individual scrolls and learn to count and group them - you will meet the aggregate functions (

COUNT

SUM

AVG

) and

GROUP BY

. The bridge is behind us, onward, deeper into the Library!

Go to CodeWorlds→