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3.3.4.2 Selecting Particular Rows

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As shown in the preceding section, it is easy to retrieve an entire table. Just omit the WHERE clause from the SELECT statement. But typically you don't want to see the entire table, particularly when it becomes large. Instead, you're usually more interested in answering a particular question, in which case you specify some constraints on the information you want. Let's look at some selection queries in terms of questions about your pets that they answer.

You can select only particular rows from your table. For example, if you want to verify the change that you made to Bowser's birth date, select Bowser's record like this:

  1. mysql> SELECT * FROM pet WHERE name = 'Bowser';
  2. +--------+-------+---------+------+------------+------------+
  3. | name   | owner | species | sex  | birth      | death      |
  4. +--------+-------+---------+------+------------+------------+
  5. | Bowser | Diane | dog     | m    | 1989-08-31 | 1995-07-29 |
  6. +--------+-------+---------+------+------------+------------+

The output confirms that the year is correctly recorded as 1989, not 1979.

String comparisons normally are case-insensitive, so you can specify the name as 'bowser', 'BOWSER', and so forth. The query result is the same.

You can specify conditions on any column, not just name. For example, if you want to know which animals were born during or after 1998, test the birth column:

  1. mysql> SELECT * FROM pet WHERE birth >= '1998-1-1';
  2. +----------+-------+---------+------+------------+-------+
  3. | name     | owner | species | sex  | birth      | death |
  4. +----------+-------+---------+------+------------+-------+
  5. | Chirpy   | Gwen  | bird    | f    | 1998-09-11 | NULL  |
  6. | Puffball | Diane | hamster | f    | 1999-03-30 | NULL  |
  7. +----------+-------+---------+------+------------+-------+

You can combine conditions, for example, to locate female dogs:

  1. mysql> SELECT * FROM pet WHERE species = 'dog' AND sex = 'f';
  2. +-------+--------+---------+------+------------+-------+
  3. | name  | owner  | species | sex  | birth      | death |
  4. +-------+--------+---------+------+------------+-------+
  5. | Buffy | Harold | dog     | f    | 1989-05-13 | NULL  |
  6. +-------+--------+---------+------+------------+-------+

The preceding query uses the AND logical operator. There is also an OR operator:

  1. mysql> SELECT * FROM pet WHERE species = 'snake' OR species = 'bird';
  2. +----------+-------+---------+------+------------+-------+
  3. | name     | owner | species | sex  | birth      | death |
  4. +----------+-------+---------+------+------------+-------+
  5. | Chirpy   | Gwen  | bird    | f    | 1998-09-11 | NULL  |
  6. | Whistler | Gwen  | bird    | NULL | 1997-12-09 | NULL  |
  7. | Slim     | Benny | snake   | m    | 1996-04-29 | NULL  |
  8. +----------+-------+---------+------+------------+-------+

AND and OR may be intermixed, although AND has higher precedence than OR. If you use both operators, it is a good idea to use parentheses to indicate explicitly how conditions should be grouped:

  1. mysql> SELECT * FROM pet WHERE (species = 'cat' AND sex = 'm')
  2.        OR (species = 'dog' AND sex = 'f');
  3. +-------+--------+---------+------+------------+-------+
  4. | name  | owner  | species | sex  | birth      | death |
  5. +-------+--------+---------+------+------------+-------+
  6. | Claws | Gwen   | cat     | m    | 1994-03-17 | NULL  |
  7. | Buffy | Harold | dog     | f    | 1989-05-13 | NULL  |
  8. +-------+--------+---------+------+------------+-------+

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Document created the 26/06/2006, last modified the 26/10/2018
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