Understanding SQL Joins - Keeping It Simple

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We'll start with something simple. The general format for a SELECT statement is:

SELECT <column list> FROM <table list> WHERE <condition list>;

Here's an example of how it can be used - the following SQL query retrieves all the records in table "a".

SELECT * FROM a;

Here's the output:

+----+------+
| a1 | a2   |
+----+------+
| 10 | u    |
| 20 | v    |
| 30 | w    |
| 40 | x    |
| 50 | y    |
| 60 | z    |
+----+------+

I can filter out some of these records by adding a WHERE clause:

SELECT * FROM a WHERE a1 > 20;

This returns

+----+------+
| a1 | a2   |
+----+------+
| 30 | w    |
| 40 | x    |
| 50 | y    |
| 60 | z    |
+----+------+

And I can even restrict the number of columns shown, by specifying a list of column names instead of the all-purpose asterisk:

SELECT a2 FROM a WHERE a1 > 20;

Which gives me:

+------+
| a2   |
+------+
| w    |
| x    |
| y    |
| z    |
+------+

Next: Crossed Wires >>
 

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