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| 在本教程中，我们将向您展示如何使用 MySQL `CASE` 表达式来构造条件查询。 | ||
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| ## MySQL CASE 表达式简介 | ||
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| MySQL `CASE` 表达式是一个流控制结构，允许您在查询中构造条件，例如：SELECT 或 WHERE 子句。 MySQL 为您提供了两种形式的 `CASE` 表达式。 | ||
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| 以下说明了 `CASE` 表达式的第一种形式。 | ||
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| ```sql | ||
| CASE value | ||
| WHEN compare_value_1 THEN result_1 | ||
| WHEN compare_value_2 THEN result_2 | ||
| … | ||
| ELSE result END | ||
| ``` | ||
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| 如果 `value` 等于 `compare_value`，例如 `compare_value_1`，`compare_value_2` 等，则 `CASE` 表达式返回相应的结果，即 `result_1`，`result_2`。 如果值不与任何 `compare_value` 匹配，则 `CASE` 表达式将返回 `ELSE` 子句中指定的结果。 | ||
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| `CASE` 表达式的第二种形式如下： | ||
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| ```sql | ||
| CASE | ||
| WHEN condition_1 THEN result_1 | ||
| WHEN condition_2 THEN result_2 | ||
| … | ||
| ELSE result END | ||
| ``` | ||
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| 在第二种形式中，如果条件为 `True`，则 `CASE` 表达式返回结果，如 `result_1`，`result_2` 等。 如果所有条件都为 `false`，则返回 `ELSE` 部分中的结果。如果省略 `ELSE` 部分，`CASE` 表达式将返回 `NULL`。 | ||
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| `CASE` 表达式返回的数据类型取决于使用它的上下文的结果。 例如，如果在字符串上下文中使用 `CASE` 表达式，则会以字符串形式返回结果。 如果在数值上下文中使用 `CASE` 表达式，则会以整数，小数或实数值的形式返回结果。 | ||
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| ## MySQL CASE 函数示例 | ||
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| 让我们来看看示例数据库 (yiibaidb) 中的 `customers` 表，其结构如下所示 - | ||
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| ```sql | ||
| mysql> desc customers; | ||
| +------------------------+---------------+------+-----+---------+-------+ | ||
| | Field | Type | Null | Key | Default | Extra | | ||
| +------------------------+---------------+------+-----+---------+-------+ | ||
| | customerNumber | int(11) | NO | PRI | NULL | | | ||
| | customerName | varchar(50) | NO | | NULL | | | ||
| | contactLastName | varchar(50) | NO | | NULL | | | ||
| | contactFirstName | varchar(50) | NO | | NULL | | | ||
| | phone | varchar(50) | NO | | NULL | | | ||
| | addressLine1 | varchar(50) | NO | | NULL | | | ||
| | addressLine2 | varchar(50) | YES | | NULL | | | ||
| | city | varchar(50) | NO | | NULL | | | ||
| | state | varchar(50) | YES | | NULL | | | ||
| | postalCode | varchar(15) | YES | | NULL | | | ||
| | country | varchar(50) | NO | | NULL | | | ||
| | salesRepEmployeeNumber | int(11) | YES | MUL | NULL | | | ||
| | creditLimit | decimal(10,2) | YES | | NULL | | | ||
| +------------------------+---------------+------+-----+---------+-------+ | ||
| 13 rows in set | ||
| ``` | ||
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| 假设您要按状态对客户进行排序，如果状态为 `NULL`，则要使用国家作为排序标准。要实现这一点，您可以使用第一种形式的 `CASE` 表达式如下： | ||
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| ```sql | ||
| SELECT | ||
| customerName, state, country | ||
| FROM | ||
| customers | ||
| ORDER BY (CASE | ||
| WHEN state IS NULL THEN country | ||
| ELSE state | ||
| END); | ||
| ``` | ||
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| 执行上面查询语句，得到以下结果 - | ||
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| ```sql | ||
| +------------------------------------+---------------+--------------+ | ||
| | customerName | state | country | | ||
| +------------------------------------+---------------+--------------+ | ||
| | Salzburg Collectables | NULL | Austria | | ||
| | Mini Auto Werke | NULL | Austria | | ||
| | Canadian Gift Exchange Network | BC | Canada | | ||
| | Royal Canadian Collectables, Ltd. | BC | Canada | | ||
| | Petit Auto | NULL | Belgium | | ||
| | Royale Belge | NULL | Belgium | | ||
| | Mini Gifts Distributors Ltd. | CA | USA | | ||
| | Mini Wheels Co. | CA | USA | | ||
| ************ 此处省略了一大波数据 ************************************* | ||
| | AV Stores, Co. | NULL | UK | | ||
| | UK Collectables, Ltd. | NULL | UK | | ||
| | Stylish Desk Decors, Co. | NULL | UK | | ||
| | Double Decker Gift Stores, Ltd | NULL | UK | | ||
| | Australian Collectors, Co. | Victoria | Australia | | ||
| | Australian Collectables, Ltd | Victoria | Australia | | ||
| +------------------------------------+---------------+--------------+ | ||
| 122 rows in set | ||
| ``` | ||
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| 在本示例中，我们使用 ORDER BY 子句中的 `CASE` 表达式来确定要排序的字段为： `state` 或 `country` 。 | ||
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| 在接下来的一个示例中，我们将使用示例数据库 (yiibaidb) 中的 `orders` 表来演示 `CASE` 表达式的第二种形式。 | ||
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| 如果您希望通过按状态查看销售订单数量，例如发货订单数量，待发货订单等，则可以使用 `CASE` 表达式的第二种形式，如下所示： | ||
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| ```sql | ||
| SELECT | ||
| SUM(CASE | ||
| WHEN status = 'Shipped' THEN 1 | ||
| ELSE 0 | ||
| END) AS 'Shipped', | ||
| SUM(CASE | ||
| WHEN status = 'On Hold' THEN 1 | ||
| ELSE 0 | ||
| END) AS 'On Hold', | ||
| SUM(CASE | ||
| WHEN status = 'In Process' THEN 1 | ||
| ELSE 0 | ||
| END) AS 'In Process', | ||
| SUM(CASE | ||
| WHEN status = 'Resolved' THEN 1 | ||
| ELSE 0 | ||
| END) AS 'Resolved', | ||
| SUM(CASE | ||
| WHEN status = 'Cancelled' THEN 1 | ||
| ELSE 0 | ||
| END) AS 'Cancelled', | ||
| SUM(CASE | ||
| WHEN status = 'Disputed' THEN 1 | ||
| ELSE 0 | ||
| END) AS 'Disputed', | ||
| COUNT(*) AS Total | ||
| FROM | ||
| orders; | ||
| ``` | ||
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| 执行上面查询语句，得到以下结果 - | ||
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| ```sql | ||
| +---------+---------+------------+----------+-----------+----------+-------+ | ||
| | Shipped | On Hold | In Process | Resolved | Cancelled | Disputed | Total | | ||
| +---------+---------+------------+----------+-----------+----------+-------+ | ||
| | 303 | 4 | 7 | 4 | 6 | 3 | 327 | | ||
| +---------+---------+------------+----------+-----------+----------+-------+ | ||
| 1 row in set | ||
| ``` | ||
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| 在 SELECT 语句中，如果状态等于 `Shipped`，`On Hold` 等，则 `CASE` 表达式返回 `1`，否则返回 `0`，我们使用 `SUM` 函数计算每种状态的订单的销售总数。 | ||
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| 在本教程中，我们向您展示了如何在 `SELECT` 语句中使用 MySQL `CASE` 表达式来构造条件查询。 |
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| 在本教程中，您将学习如何使用 MySQL `CURDATE()` 函数获取当前日期。 | ||
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| ## MySQL CURDATE 功能介绍 | ||
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| 如果在数字上下文中使用字符串上下文或 `YYYMMDD` 格式，`CURDATE()` 函数将以“`YYYY-MM-DD`”格式的值返回当前日期。 | ||
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| 以下示例显示了如何在字符串上下文中使用 `CURDATE()` 函数。 | ||
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| ```sql | ||
| sql> SELECT CURDATE(); | ||
| +------------+ | ||
| | CURDATE() | | ||
| +------------+ | ||
| | 2017-08-10 | | ||
| +------------+ | ||
| 1 row in set (0.00 sec) | ||
| ``` | ||
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| 以下示例说明 `CURDATE()` 函数如何在数字上下文中使用： | ||
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| ```sql | ||
| mysql> SELECT CURDATE() + 0; | ||
| +---------------+ | ||
| | CURDATE() + 0 | | ||
| +---------------+ | ||
| | 20170810 | | ||
| +---------------+ | ||
| 1 row in set | ||
| ``` | ||
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| `CURRENT_DATE` 和 `CURRENT_DATE()` 是 `CURDATE()` 的同义词。 | ||
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| ```sql | ||
| mysql> SELECT CURRENT_DATE(), CURRENT_DATE, CURDATE(); | ||
| +----------------+--------------+------------+ | ||
| | CURRENT_DATE()| CURRENT_DATE | CURDATE() | | ||
| +----------------+--------------+------------+ | ||
| | 2017-08-10 | 2017-08-10 | 2017-08-10 | | ||
| +----------------+--------------+------------+ | ||
| 1 row in set | ||
| ``` | ||
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| ## CURDATE 与 NOW 函数 | ||
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| `CURDATE()` 函数只返回当前日期，而 NOW() 函数返回当前时间的日期和时间部分。`CURDATE()` 函数的结果等同于以下表达式： | ||
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| ```sql | ||
| mysql> SELECT DATE(NOW()); | ||
| +-------------+ | ||
| | DATE(NOW()) | | ||
| +-------------+ | ||
| | 2017-08-10 | | ||
| +-------------+ | ||
| 1 row in set | ||
| ``` | ||
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| 在本教程中，您学习了如何使用 MySQL `CURDATE()` 函数获取当前日期值。 |
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