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Home php教程 php手冊(cè) PHP中操作MySQL數(shù)據(jù)庫的注意事項(xiàng)

PHP中操作MySQL數(shù)據(jù)庫的注意事項(xiàng)

Jun 21, 2016 am 09:15 AM
mysql name null quot

mysql|數(shù)據(jù)|數(shù)據(jù)庫

1. 分號(hào)的例外
 對(duì)于 MySQL ,第一件你必須牢記的是它的每一行命令都是用分號(hào) (;) 作為結(jié)束的,但……沒有完全絕對(duì)的事,在這兒也是一樣,當(dāng)一行 MySQL 被插入在 PHP 代碼中時(shí),最好把后面的分號(hào)省略掉,例如:
mysql_query ("INSERT INTO tablename (first_name, last_name)
VALUES ('$first_name', '$last_name')
");
 這是因?yàn)?PHP 也是以分號(hào)作為一行的結(jié)束的,額外的分號(hào)有時(shí)會(huì)讓 PHP 的語法分析器搞不明白,所以還是省略掉的好。在這種情況下,雖然省略了分號(hào),但是 PHP 在執(zhí)行 MySQL 命令時(shí)會(huì)自動(dòng)的幫你加上的。

 另外還有一個(gè)不要加分號(hào)的情況。當(dāng)你想把要字段的豎者排列顯示下來,而不是像通常的那樣橫著排列時(shí),你可以用 \G 來結(jié)束一行 SQL 語句,這時(shí)就用不上分號(hào)了,例如:

SELECT * FROM PENPALS WHERE USER_ID = 1\G

2. TEXT、DATE、和 SET 數(shù)據(jù)類型

 MySQL 數(shù)據(jù)表的字段必須有定義一個(gè)數(shù)據(jù)類型。這有大約 25 種選擇,大部分都是直接明了的,就不多費(fèi)口舌了。但有幾個(gè)有必要提一下。

 TEXT 不是一種數(shù)據(jù)類型,雖然可能有些書上是這么說的。它實(shí)際上應(yīng)該是“ LONG VARCHAR ”或者“ MEDIUMTEXT ”。

 DATE 數(shù)據(jù)類型的格式是 YYYY-MM-DD ,比如: 1999-12-08 。你可以很容易的用 date 函數(shù)來得到這種格式的當(dāng)前系統(tǒng)時(shí)間:

date("Y-m-d")

 并且,在 DATA 數(shù)據(jù)類型之間可以作減法,得到相差的時(shí)間天數(shù):

$age = ($current_date - $birthdate);

 集合 SET 是一個(gè)有用的數(shù)據(jù)類型,它和枚舉 ENUM 有點(diǎn)相似,只不過是 SET 能夠保存多個(gè)值而 ENUM 只能保存一個(gè)值而已。而且, SET 類型最多只能夠有 64 個(gè)預(yù)定的值,而 ENUM 類型卻能夠處理最多 65,535 個(gè)預(yù)定義的值。而如果需要有大于 64 個(gè)值的集合,該怎么辦呢?這時(shí)就需要定義多個(gè)集合來一起解決這個(gè)問題了。

3. 通配符

 SQL 的通配符有兩種:“ * ”和“ % ”。分別用在不同的情況下。例如:如果你想看到數(shù)據(jù)庫的所有內(nèi)容,可以像這樣來查詢:

SELECT * FROM dbname WHERE USER_ID LIKE '%';

 這兒,兩個(gè)通配符都被用上了。他們表示相同的意思 ?? 都是用來匹配任何的字符串,但是他們用在不同的上下文中?!?* ”用來匹配字段名,而“ % ”用來匹配字段值。另外一個(gè)不容易引起注意的地方是“ % ”通配符需要和 LIKE 關(guān)鍵字一起使用。

還有一個(gè)通配符,就是下劃線“ _ ”,它代表的意思和上面不同,是用來匹配任何單個(gè)的字符的。

4. NOT NULL 和空記錄

 如果用戶在沒有填任何東西的情況下按了 submit 按鈕,會(huì)怎樣呢?如果你確實(shí)需要一個(gè)值,那么可以用客戶端腳本或者服務(wù)器端腳本來進(jìn)行數(shù)據(jù)驗(yàn)證,這一點(diǎn)在前面已經(jīng)說過了。但是,在數(shù)據(jù)庫中卻是允許一些字段被空出來什么也不填。對(duì)此類紀(jì)錄, MySQL 將要為之執(zhí)行一些事情:

插入值 NULL ,這是缺省的操作。
如果你在字段定義中為之聲明了 NOT NULL (在建立或者修改這個(gè)字段的時(shí)候), MySQL 將把這個(gè)字段空出來什么東西也不填。
對(duì)于一個(gè) ENUM 枚舉類型的字段,如果你為之聲明了 NOT NULL , MySQL 將把枚舉集的第一個(gè)值插入到字段中。也就是說, MySQL 把枚舉集的第一個(gè)值作為這個(gè)枚舉類型的缺省值。

 一個(gè)值為 NULL 的紀(jì)錄和一個(gè)空紀(jì)錄是有一些區(qū)別的。 % 通配符可以匹配空紀(jì)錄,但是卻不能匹配 NULL 紀(jì)錄。在某些時(shí)候,這種區(qū)別會(huì)造成一些意想不到的后果。就我的經(jīng)驗(yàn)而言,任何字段都應(yīng)該聲明為 NOT NULL 。這樣下面的 SELECT 查詢語句就能夠正常運(yùn)轉(zhuǎn)了:

if (!$CITY) {$CITY = "%";}

$selectresult = mysql_query ("SELECT * FROM dbname
WHERE FIRST_NAME = ' 柳 '
AND LAST_NAME = ' 如風(fēng) '
AND CITY LIKE '$CITY'
");

 在第一行中,如果用戶沒有指定一個(gè) CITY 值,那么就會(huì)用通配符 % 來代入 CITY 變量,這樣搜索時(shí)就會(huì)把任何的 CITY 值都考慮進(jìn)去,甚至包括那些 CITY 字段為空的紀(jì)錄。

 但是如果有一些紀(jì)錄,它的 CITY 字段值是 NULL ,這時(shí)問題就出現(xiàn)了。上面的查詢是不能夠找到這些字段的。問題的一個(gè)解決辦法可以是這樣:

if (!$CITY) {$CITY = "%";}

$selectresult = mysql_query ("SELECT * FROM dbname
WHERE FIRST_NAME = ' 柳 '
AND LAST_NAME = ' 如風(fēng) '
AND (CITY LIKE '$CITY' OR CITY IS NULL)
");

 注意在搜索 NULL 時(shí),必須用“ IS ”關(guān)鍵字,而 LIKE 時(shí)不會(huì)正常工作的。

 在最后要提到的是,如果你在加入或者修改一個(gè)新的字段之前,數(shù)據(jù)庫中已經(jīng)有了一些記錄了,這時(shí)新加入的字段在原來的紀(jì)錄中的值,可能是 NULL ,也可能為空。這也算是 MySQL 的一個(gè) Bug 吧,所以在這種情況下,使用 SELECT 查詢要特別的小心。



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