Learning Mysql

You can combine
AND
and OR, but you need to make it clear whether you want to first
AND
the conditions or
OR
them. Consider an example where the function isn’t obvious from the query:
mysql> SELECT album_name FROM album WHERE
-> album_name LIKE "L" OR
-> album_name LIKES" AND
-> album_name LIKE "%g";
+-----------------------+
| album_name |
+-----------------------+
| Let Love In |
| Live Around The World |
| Second Coming |
| Light Years |
+-----------------------+
4 rows inset sec)
When you inspect the results, it becomes clear what’s happened the answers either begin with L, or they have Sat the beginning and g
at the end. An alternative interpretation of the query would be that the answers must begin with
L
or Sand all end with g this is clearly not how the MySQL server has handled the query, since one of the displayed answers, Let Love In doesn’t end in a g. To make queries containing several
Boolean conditions easier to read, group conditions within parentheses.
Parentheses cluster parts of a statement together and help make expressions readable;
you can use them just as you would in basic math. Our previous example can be rewritten as follows:
mysql> SELECT album_name FROM album WHERE
-> album_name LIKE "L" OR
-> (album_name LIKES" AND album_name LIKE "g
+-----------------------+
| album_name |
+-----------------------+
| Let Love In |
| Live Around The World |
| Second Coming |
| Light Years |
+-----------------------+
4 rows inset sec)
The parentheses make the evaluation order clear we want albums beginning with 'L, or those beginning with 'Sand ending with 'g. We’ve also typed the query over three lines instead of four, making the intention even clearer through careful layout;
just as when writing program code, spacing, indentation, and careful layout help make readable queries.
You can also use parentheses to force a different evaluation order. If you did want albums having names with 'L'
or 'Sat the beginning and 'g'
at the end, you’d type:
mysql> SELECT album_name FROM album WHERE

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