[SQL First Step] 6. Specifying Conditions

SELECT is used to Specify Columns.

SELECT is used to speicfy columns. Use as follows.

SELECT col1, col2 FROM table_name

mysql> SELECT no, name FROM sample21;
+------+-----------+
| no   | name      |
+------+-----------+
|    1 | 박준용    |
|    2 | 김재진    |
|    3 | 홍길동    |
+------+-----------+

There is no need to write columns in order exactly same as database. Write down in order you want. Repeating the same columns also works fine.

WHERE is used to Specify Rows.

To search desired rows, use WHERE to set a condition.

SELECT col1, col2 FROM table_name WHERE condition

Unlike SELECT and FROM, WHERE is not a essential phrase. However, the format and order above must be obeyed.

The followings are examples of WHERE.

mysql> SELECT * FROM sample21 WHERE no=2;
+------+-----------+----------+------------------------+
| no   | name      | birthday | address                |
+------+-----------+----------+------------------------+
|    2 | 김재진    | NULL     | 대구광역시 동구        |
+------+-----------+----------+------------------------+
1 row in set (0.00 sec)

mysql> SELECT * FROM sample21 WHERE no<>2;
+------+-----------+------------+---------------------------+
| no   | name      | birthday   | address                   |
+------+-----------+------------+---------------------------+
|    1 | 박준용    | 1976-10-18 | 대구광역시 수성구         |
|    3 | 홍길동    | NULL       | 서울특별시 마포구         |
+------+-----------+------------+---------------------------+
2 rows in set (0.00 sec)

mysql> SELECT * FROM sample21 WHERE name='박준용';
+------+-----------+------------+---------------------------+
| no   | name      | birthday   | address                   |
+------+-----------+------------+---------------------------+
|    1 | 박준용    | 1976-10-18 | 대구광역시 수성구         |
+------+-----------+------------+---------------------------+
1 row in set (0.00 sec)

mysql> SELECT * FROM sample21 WHERE birthday IS NULL;
+------+-----------+----------+---------------------------+
| no   | name      | birthday | address                   |
+------+-----------+----------+---------------------------+
|    2 | 김재진    | NULL     | 대구광역시 동구           |
|    3 | 홍길동    | NULL     | 서울특별시 마포구         |
+------+-----------+----------+---------------------------+
2 rows in set (0.00 sec)

There are several operators in SQL.

Operators	Meanings
=	True if the left and right side are the same.
<>	True if the left and right side are not the same.
>	True if the left is greater than the right side.
<	True if the left is less than the right side.
>=	True if the left side is greater than or equal to the right side.
<=	True if the left side is less than or equal to the right side.

There are also a few key points.

• Use ‘’ for character type, date type and time type.
• For NULL, use IS NULL or IS NOT NULL. = and <> will not work.

All images, except those with separate source indications, are excerpted from lecture materials.