我們在 Day 12 學習了如何選取資料表的資料，並且了解可以選擇特定欄位輸出。

這次我們使用 racehorse 當作範例，我們先把所有資料撈出來：

SELECT * FROM racehorse;

我們可以選擇想看的欄位，比如這次我們只想看id, name, birth_date, coat_color，我們可以這樣寫：

SELECT id, name, birth_date, coat_color FROM racehorse;

 id |     name      | birth_date | coat_color
----+---------------+------------+------------
  1 | Gold Ship     | 2009-03-06 | Grey
  2 | Vodka         | 2004-04-04 | Bay
  3 | Daiwa Scarlet | 1995-02-18 | Chestnut
  4 | Grass Wonder  | 1995-02-18 | Chestnut
  5 | Oguri Cap     | 1985-03-27 | Gray

使用 WHERE 設定條件

但我們有時候並不需要全部的資料，有時候我們只想看特定條件的資料。
比如，我只對毛色（coat_color）為栗色（Chestnut）的賽馬有興趣，我們就可以使用 WHERE 定義我們想要的條件。

先來看 WHERE 的 SQL 語法：

SELECT * FROM table_name WHERE <condition>;

我們可以這樣寫：

SELECT id, name, birth_date, coat_color
FROM racehorse
WHERE coat_color = 'Chestnut';

這樣就會輸出我們想看的資料了。

當然，WHERE 當然不是只有 = 可以使用。
如同其他程式語言， SQL 有許多比較運算子可以運用。

參考如下，引用自How to filter query results in PostgreSQL：

• = equal to
• > greater than
• < less than
• >= greater than or equal to
• <= less than or equal to
• <> or != not equal
• AND the logical "and" operator — joins two conditions and returns TRUE if both of the conditions are TRUE
• OR logical "or" operator — joins two conditions and returns TRUE if at least one of the conditions are TRUE
• IN value is contained in the list, series, or range that follows
• BETWEEN value is contained within the range the minimum and maximum values that follow, inclusive
• IS NULL matches if value is NULL
• NOT negates the boolean value that follows
• EXISTS the query that follows contains results
• LIKE matches against a pattern (using the wildcards % to match 0 or more characters and _ to match a single character)
• ILIKE matches against a pattern (using the wildcards % to match 0 or more characters and _ to match a single character), case insensitive
• SIMILAR TO matches against a pattern using SQL's regular expression dialect
• ~ matches against a pattern using POSIX regular expressions, case sensitive
• ~* matches against a pattern using POSIX regular expressions, case insensitive
• !~ does not match against a pattern using POSIX regular expressions, case sensitive
• !~* does not match against a pattern using POSIX regular expressions, case insensitive

參考資料：

• How to filter query results in PostgreSQL