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高精度的科學或製造數據通常要求精確到小數點後8位。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"爲了處理這些類型的數據（以及更多類型的數據），MySQL提供幾種不同的數值數據類型。最常用的是用來存儲整數即integers的類型。在指定類型的時候，還可以指定數據是unsigned的，以向服務器指明存儲這列的所有數據都將大於或等於零。下表（2-2）展示了用於存儲整數的五種不同數據類型：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/54/5450ae71699abf100e7a27800c3f5033.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" 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預計未來發生的特定事件的日期（例如運送客戶訂單）；","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• 客戶訂單的實際發貨日期；","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• 用戶修改表中某行的日期和時間；","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• 員工的出生日期；","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• 與數據倉庫的yearly_sales表中，每行對應的年份；","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• 在汽車裝配線上完成流水線所需的時間。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"MySQL能爲上述所有情況提供合適的數據類型。MySQL支持的時間數據類型如下表（2-4）所示：","attrs":{}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/30/305e6fc406eaa005b58eb3f6e37eeb6b.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/d3/d3fb2bf8cecb2a5c3141315168e91d02.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" 雖然數據庫服務器能夠以各種方式存儲時間數據，但格式字符串（上表2-4的第二列）的目的是指定檢索時如何顯示數據，以及插入或更新時間列時需要提供日期字符串的格式。因此，如果要使用默認格式爲YYYY-MM-DD的date列存儲日期2020年3月23日則要使用字符串'2020-03-23'。第七章全面探討了時間數據是如何構造和顯示的。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"datetime、timestamp和time類型還允許小數點後6位（微秒）的小數秒，使用這些數據類型之一定義列時，可以提供0到6之間的值；例如，指定datetime(2)將允許時間值包含百分之一秒。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}}],"text":"注意：每個數據庫服務器針對時間列允許的日期範圍各不相同。Oracle數據庫接受從公元前4712年到公元9999年的日期，而SQL Server只能處理從公元1753年到公元9999年的日期（除非用的是SQL Server 2008的datetime2數據類型，它允許從公元1年到公元9999年的日期）。MySQL介於Oracle和SQL Server之間，可以存儲從公元1000年到公元9999年的日期。雖然對於大多數處理當前和未來事件的系統來說，這些差別可能並不重要，但存放歷史時期的時候就請務必記住這些差別了。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"下面介紹如何使用各種時間類型來實現前面的例子：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• 用於保存客戶訂單的預期未來發貨日期和員工出生日期的列將使用date類型，因爲未來發貨時間精確到秒是不現實的，此外也沒有必要知道一個人出生的具體時間。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• 保存客戶訂單實際發貨時間信息的列可以用datetime類型，因爲不僅要記錄發貨日期，還要記錄發貨時間。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• 記錄用戶修改表中某行的日期和時間，可以用使用timestamp類型。timestamp類型與datetime類型（包括年、月、日、小時、分鐘、秒）保存的信息相同，但是當在表中新增一行或修改一行時，MySQL服務器將自動爲timestamp列增加當前日期/時間信息。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• 僅包含年份數據的列將使用year類型。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• 保存完成任務所需時長數據的列將使用time類型。對於這種類型的數據，無需存儲日期信息，因爲你只關心完成任務所需的時/分/秒數。該類信息可以使用兩個datetime列來存儲（一個用於存放任務開始日期/時間，另一個用於存放任務完成日期/時間），兩者的差值就是時長，但是很顯然，使用單個time列會更簡單。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"第七章會探討如何使用這些時間數據類型。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"創建表","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"現在你已經瞭解了MySQL數據庫中可能存儲的數據類型，所以是時候看看如何在表定義中使用這些類型了。讓我們先定義一個表來保存個人信息。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"第一步：設計","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"開始設計表格之前，最好是頭腦風暴一下，以確定需要包括的信息。以下是我在思考了一會兒之後想出的用於描述個人信息的項：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• 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name列實際上是一個由名字和姓氏組成的複合對象。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• 由於多個人可以擁有相同的姓名、眼睛顏色、出生日期等，因此person表中沒有保證唯一性的列。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• address列也是一個複合對象，由街道、城市、州/省、國家和郵政編碼組成。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• favorite_foods列是一個包含零個、一個或多個獨立條目的列表，所以最好爲這些數據創建一個單獨的表，然後在其中包含person表的外鍵，這樣就可以知道某個特定食物歸屬的人員了。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"考慮到以上問題，下表（2-7）給出了person表的規範化版本：","attrs":{}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/b1/b139881316d8654b4043d0e0bfa606b0.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"既然person表有主鍵（person_id）來保證唯一性，那麼下一步就是構建一個favorite_food表，其中包含指向person表的外鍵，如下表（2-8）所示：","attrs":{}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/22/222be70841c6bd1c738b9671eaddb0b5.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"person_id和food列構成favorite_food表的主鍵，person_id列也是person表的外鍵。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"horizontalrule","attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"這些設計就夠了嗎？","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"將favorite_foods列從person表中移出絕對是個好主意，但這樣就夠了嗎？例如，如果一個人把pasta列爲最喜歡的食物，而另一個人卻把spaghetti列爲最喜歡的食物，它們指的都是意大利麪，那該怎麼辦？它們是一樣的嗎？爲了防止此問題發生，你可以讓人們從列表中選擇他們最喜歡的食物，在這種情況下，應該創建一個包含food_id和food_name列的food表，然後修改favorite_food包含food表的外鍵。雖然這種設計是完全規範化的，但若你只是想存儲用戶輸入的值，那麼可以保持原有的表設計。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"horizontalrule","attrs":{}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"第三步：構建SQL語句","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"現在已經完成了這兩個表的設計，它們包括關於人員信息以及他們喜歡的食物的信息，下一步是生成SQL語句來在數據庫中創建表。下面是創建person表的語句：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"CREATE","attrs":{}},{"type":"text","text":" 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使用文本類型時，應注意以下幾點：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• 如果裝載到文本列中的數據超過該類型的最大大小，數據將被截斷。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• 將數據裝載到文本列中時，不會刪除尾部的空格。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• 使用文本列進行排序或分組時，僅使用前1024個字節，但必要時可放寬該限制。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• 這些不同的文本類型是MySQL獨有的。對於大字符數據，SQLServer只有一種文本類型（text類型），而DB2和Oracle使用一種稱爲clob的數據類型（Character Large Object）。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• 既然MySQL允許varchar列最多容納65535字節（在版本4中限制爲255字節），那麼一般不需要特別使用tinytext或text類型了。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果你要創建一個用於存儲自由格式數據的列，例如一個notes列來存儲有關客戶與公司的客服部門交互的數據，那麼varchar可能就足夠了。但是，如果要存儲文檔，則應選擇mediumtext或longtext類型。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"注意：Oracle數據庫中char列最多容納2000字節，varchar2列最多容納4000字節。對於較大的文檔，可以使用clob類型。對於char和varchar數據，SQL Server最多可以處理8000個字節，但是在定義爲varchar(max)的列中最多可以存儲2GB的數據。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"數值型數據","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"儘管使用叫“numeric”的數值數據類型似乎是合理的，但實際上存在好幾種不同的數值數據類型，它們反映了使用數字的各種方式，如下所示：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• ","attrs":{}},{"type":"text","marks":[{"type":"italic","attrs":{}}],"text":"某列需要指示客戶訂單是否已發送","attrs":{}}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" 這種類型的列可以被設爲Boolean，將包含一個0或1，分別表示false和true。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• ","attrs":{}},{"type":"text","marks":[{"type":"italic","attrs":{}}],"text":"系統爲交易表生成的主鍵","attrs":{}}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" 這列數據通常從1開始，每次自增1，最後的數字可能會非常大。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• ","attrs":{}},{"type":"text","marks":[{"type":"italic","attrs":{}}],"text":"顧客電子購物籃的商品編號","attrs":{}}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" 這類列的值應該是一個正整數，範圍從1到200（假設購物狂的購物籃能容納最多的物品數量是200，當然也可能更大或更小）。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• ","attrs":{}},{"type":"text","marks":[{"type":"italic","attrs":{}}],"text":"電路板鑽牀的位置數據","attrs":{}}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" 高精度的科學或製造數據通常要求精確到小數點後8位。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"爲了處理這些類型的數據（以及更多類型的數據），MySQL提供幾種不同的數值數據類型。最常用的是用來存儲整數即integers的類型。在指定類型的時候，還可以指定數據是unsigned的，以向服務器指明存儲這列的所有數據都將大於或等於零。下表（2-2）展示了用於存儲整數的五種不同數據類型：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/54/5450ae71699abf100e7a27800c3f5033.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" ","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"當你使用其中一個整數類型創建列時，MySQL將爲存儲數據分配適當大小的空間，範圍從tinyint的一個字節到bigint的八個字節。因此在選擇數據類型的時候，確保能夠容納你所預期的最大數據即可，以免白白浪費存儲空間。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"對於浮點數（如3.1415927），可以從下表（2-3）所示的數字類型中選擇：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/b5/b549ebb707780f74c67362dfad63ebc0.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"italic","attrs":{}}],"text":" ","attrs":{}},{"type":"text","text":"使用浮點類型時，可以指定精度precision（小數點左側到右側允許的總位數）和有效位scale（小數點右側允許的位數），不過這並不是必須的。上面兩個值在上表（2-3）中表示爲p和s。注意：爲浮點類型的列指定精度和有效位的時候，如果位數超過該列定義的小數位數和精度，則存儲在該列中的數據將被四捨五入。例如，定義爲float(4,2)的列總共會存儲四位數字，兩位在小數點左側，兩位在小數點右側。因此，這樣的列可以很好地存儲27.44和8.19這兩個數字，但是存儲17.8675將被四捨五入到17.87，存儲178.375則會產生錯誤。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"與整數類型一樣，浮點列可以被定義爲無符號的（unsigned），但這隻能防止列中存放負數，而無法改變列所能存儲數據的範圍。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"時間數據","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"除了字符串和數字，你肯定還會使用有關日期和時間的信息。這種類型的數據稱爲時間型數據，下面是數據庫中有關時間型數據的一些例子：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• 預計未來發生的特定事件的日期（例如運送客戶訂單）；","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• 客戶訂單的實際發貨日期；","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• 用戶修改表中某行的日期和時間；","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• 員工的出生日期；","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• 與數據倉庫的yearly_sales表中，每行對應的年份；","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• 在汽車裝配線上完成流水線所需的時間。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"MySQL能爲上述所有情況提供合適的數據類型。MySQL支持的時間數據類型如下表（2-4）所示：","attrs":{}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/30/305e6fc406eaa005b58eb3f6e37eeb6b.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/d3/d3fb2bf8cecb2a5c3141315168e91d02.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" 雖然數據庫服務器能夠以各種方式存儲時間數據，但格式字符串（上表2-4的第二列）的目的是指定檢索時如何顯示數據，以及插入或更新時間列時需要提供日期字符串的格式。因此，如果要使用默認格式爲YYYY-MM-DD的date列存儲日期2020年3月23日則要使用字符串'2020-03-23'。第七章全面探討了時間數據是如何構造和顯示的。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"datetime、timestamp和time類型還允許小數點後6位（微秒）的小數秒，使用這些數據類型之一定義列時，可以提供0到6之間的值；例如，指定datetime(2)將允許時間值包含百分之一秒。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}}],"text":"注意：每個數據庫服務器針對時間列允許的日期範圍各不相同。Oracle數據庫接受從公元前4712年到公元9999年的日期，而SQL Server只能處理從公元1753年到公元9999年的日期（除非用的是SQL Server 2008的datetime2數據類型，它允許從公元1年到公元9999年的日期）。MySQL介於Oracle和SQL Server之間，可以存儲從公元1000年到公元9999年的日期。雖然對於大多數處理當前和未來事件的系統來說，這些差別可能並不重要，但存放歷史時期的時候就請務必記住這些差別了。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"下面介紹如何使用各種時間類型來實現前面的例子：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• 用於保存客戶訂單的預期未來發貨日期和員工出生日期的列將使用date類型，因爲未來發貨時間精確到秒是不現實的，此外也沒有必要知道一個人出生的具體時間。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• 保存客戶訂單實際發貨時間信息的列可以用datetime類型，因爲不僅要記錄發貨日期，還要記錄發貨時間。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• 記錄用戶修改表中某行的日期和時間，可以用使用timestamp類型。timestamp類型與datetime類型（包括年、月、日、小時、分鐘、秒）保存的信息相同，但是當在表中新增一行或修改一行時，MySQL服務器將自動爲timestamp列增加當前日期/時間信息。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• 僅包含年份數據的列將使用year類型。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• 保存完成任務所需時長數據的列將使用time類型。對於這種類型的數據，無需存儲日期信息，因爲你只關心完成任務所需的時/分/秒數。該類信息可以使用兩個datetime列來存儲（一個用於存放任務開始日期/時間，另一個用於存放任務完成日期/時間），兩者的差值就是時長，但是很顯然，使用單個time列會更簡單。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"第七章會探討如何使用這些時間數據類型。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"創建表","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"現在你已經瞭解了MySQL數據庫中可能存儲的數據類型，所以是時候看看如何在表定義中使用這些類型了。讓我們先定義一個表來保存個人信息。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"第一步：設計","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"開始設計表格之前，最好是頭腦風暴一下，以確定需要包括的信息。以下是我在思考了一會兒之後想出的用於描述個人信息的項：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• 姓名","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• 眼睛顏色","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• 出生日期","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• 地址","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• 喜愛的食物","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"當然這些信息並不全面，但是這裏已經夠用了。下一步是分配列名和數據類型。下表（2-6）是我的初始設計：","attrs":{}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/95/955d8f3c42c8a5c0ba0281b552336669.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"name、address和favorite_foods列的類型爲varchar，允許任何格式的數據條目。eye_color列限制兩個字符只應爲BR、BL或GR中的一個。birth_date列的類型爲date，因爲不需要包括具體的時間信息。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"第二步：完善","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在第一章中，我們介紹過規範化的概念——規範化是確保數據庫設計中沒有重複（外鍵除外）或複合列的過程。再次查看person表中的列時，會發現以下問題：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"• 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