對(duì)于程序中的一般字符串類(lèi)型的字段，SQL Server中有char、varchar、nchar、nvarchar四種類(lèi)型來(lái)對(duì)應(yīng)，那么這四種類(lèi)型有什么區(qū)別呢，這里做一下對(duì)比。
1.定長(zhǎng)或變長(zhǎng)
所謂定長(zhǎng)就是長(zhǎng)度固定，當(dāng)要保存的數(shù)據(jù)長(zhǎng)度不夠時(shí)將自動(dòng)在其后面填充英文空格，使長(zhǎng)度達(dá)到相應(yīng)的長(zhǎng)度；有var前綴的，表示是實(shí)際存儲(chǔ)空間是動(dòng)態(tài)變化的，比如varchar,nvarchar變長(zhǎng)字符數(shù)據(jù)則不會(huì)以空格填充。
2.Unicode或非Unicode
數(shù)據(jù)庫(kù)中，英文字符只需要一個(gè)字節(jié)存儲(chǔ)就足夠了，但漢字和其他眾多非英文字符，則需要兩個(gè)字節(jié)存儲(chǔ)。如果英文與漢字同時(shí)存在，由于占用空間數(shù)不同，容易造成混亂，導(dǎo)致讀取出來(lái)的字符串是亂碼。Unicode字符集就是為了解決字符集這種不兼容的問(wèn)題而產(chǎn)生的，它所有的字符都用兩個(gè)字節(jié)表示，即英文字符也是用兩個(gè)字節(jié)表示。而前綴n就表示Unicode字符，比如nchar,nvarchar，這兩種類(lèi)型使用了Unicode字符集。
3.幾種數(shù)據(jù)類(lèi)型的存儲(chǔ)的最大容量
char，varchar 最多8000個(gè)英文，4000個(gè)漢字
nchar，nvarchar 最多可存儲(chǔ)4000個(gè)字符，無(wú)論英文還是漢字
復(fù)制代碼 代碼如下:
--創(chuàng)建表
CREATE TABLE TempTable(
id INT PRIMARY KEY,
charField CHAR(10),
varcharField VARCHAR(10),
nvarcharField NVARCHAR(10)
)
INSERT INTO TempTable VALUES(1,'WFTH','WFTH','WFTH')
INSERT INTO TempTable VALUES(2,'無(wú)風(fēng)聽(tīng)海','無(wú)風(fēng)聽(tīng)海','無(wú)風(fēng)聽(tīng)海')
INSERT INTO TempTable VALUES(3,'','','')
INSERT INTO TempTable(id) VALUES(4)
INSERT INTO TempTable VALUES(5,'1234567890','1234567890','12345')
SELECT DATALENGTH(charField) AS charFieldLen,
DATALENGTH(varcharField) AS varcharFieldLen,
DATALENGTH(nvarcharField) AS nvarcharFieldLen
FROM temptable WHERE id =1
SELECT DATALENGTH(charField) AS charFieldLen,
DATALENGTH(varcharField) AS varcharFieldLen,
DATALENGTH(nvarcharField) AS nvarcharFieldLen
FROM temptable WHERE id =2
SELECT DATALENGTH(charField) AS charFieldLen,
DATALENGTH(varcharField) AS varcharFieldLen,
DATALENGTH(nvarcharField) AS nvarcharFieldLen
FROM temptable WHERE id =3
SELECT DATALENGTH(charField) AS charFieldLen,
DATALENGTH(varcharField) AS varcharFieldLen,
DATALENGTH(nvarcharField) AS nvarcharFieldLen
FROM temptable WHERE id =4
SELECT DATALENGTH(charField) AS charFieldLen,charField,
DATALENGTH(varcharField) AS varcharFieldLen,varcharField,
DATALENGTH(nvarcharField) AS nvarcharFieldLen,nvarcharField
FROM temptable WHERE id =5
