大多數資料庫使用靜态的嚴格的類型系統,列的類型在建立表的時候就已經指定了。SQLite使用動态的類型系統,列的類型由值決定。
1.存儲類型
以下是SQLite的存儲類型,SQLite中存儲的每個值都對應以下一種存儲類型
NULL: NULL value
Integer: 值是signed integer 類型,大小可以是1,2,3,4,6,8bytes
REAL: 浮點類型
TEXT: 以UTF-8, UTF-16BE or UTF-16LE編碼存儲的字元類型
BLOB: 二進制資料
Integer primary key 列是個例外??
布爾類型: SQLite中沒有定義布爾類型,而是以Integer 存儲布爾值,0(false), 1(true)
Date and time 類型
SQLite中也沒有定義日期時間類型,日期時間可以用TEXT, REAL, or INTEGER存儲
TEXT: 存儲為字元串("YYYY-MM-DD HH:MM:SS.SSS").
REAL: as Julian day numbers, the number of days since noon in Greenwich on November 24, 4714 B.C. according to the proleptic Gregorian calendar.
INTEGER: as Unix Time, the number of seconds since 1970-01-01 00:00:00 UTC.
2. SQLite Type Affinity
用于自動檢測值的類型,以下列舉Affinity如何決定類型的規則
(1) 如果類型聲明中有int,則使用INTEGER affinity.
(2) 如果類型聲明中有"CHAR", "CLOB", or "TEXT", 則使用Text affinity
(3) 如果類型聲明中有BLOB或沒有指定類型,則使用affinity NONE
(4) 如果類型聲明中有"REAL", "FLOA", or "DOUB", 則使用 REAL affinity
(5) 否則使用Numeric affinity
SQLite 有以下幾種affinity
- TEXT
- NUMERIC
- INTEGER
- REAL
- NONE
例子
CREATE TABLE t1(
t TEXT, -- text affinity by rule 2
nu NUMERIC, -- numeric affinity by rule 5
i INTEGER, -- integer affinity by rule 1
r REAL, -- real affinity by rule 4
no BLOB -- no affinity by rule 3
);
-- Values stored as TEXT, INTEGER, INTEGER, REAL, TEXT.
INSERT INTO t1 VALUES('500.0', '500.0', '500.0', '500.0', '500.0');
SELECT typeof(t), typeof(nu), typeof(i), typeof(r), typeof(no) FROM t1;
text|integer|integer|real|text
-- Values stored as TEXT, INTEGER, INTEGER, REAL, REAL.
DELETE FROM t1;
INSERT INTO t1 VALUES(500.0, 500.0, 500.0, 500.0, 500.0);
SELECT typeof(t), typeof(nu), typeof(i), typeof(r), typeof(no) FROM t1;
text|integer|integer|real|real
-- Values stored as TEXT, INTEGER, INTEGER, REAL, INTEGER.
DELETE FROM t1;
INSERT INTO t1 VALUES(500, 500, 500, 500, 500);
SELECT typeof(t), typeof(nu), typeof(i), typeof(r), typeof(no) FROM t1;
text|integer|integer|real|integer
-- BLOBs are always stored as BLOBs regardless of column affinity.
DELETE FROM t1;
INSERT INTO t1 VALUES(x'0500', x'0500', x'0500', x'0500', x'0500');
SELECT typeof(t), typeof(nu), typeof(i), typeof(r), typeof(no) FROM t1;
blob|blob|blob|blob|blob
-- NULLs are also unaffected by affinity
DELETE FROM t1;
INSERT INTO t1 VALUES(NULL,NULL,NULL,NULL,NULL);
SELECT typeof(t), typeof(nu), typeof(i), typeof(r), typeof(no) FROM t1;
null|null|null|null|null
3. 類型比較
NULL < TEXT< BLOB < INTEGER or REAL, INTEGER,REAL之間用數學方法比較, BLOB之間用memcmp()函數比較
memcmp函數原型
int memcmp ( const void * ptr1, const void * ptr2, size_t num );
比較兩個指針指向記憶體的前num個byte
比較之前的類型轉換
- (INTEGER, REAL or NUMERIC) 和 (TEXT or NONE)比較,則TEXT, NONE會被轉換成NUMERIC
- TEXT和NONE比較,則NONE會被轉換成TEXT
- 其他情況直接比較
參考
http://www.sqlite.org/datatype3.html (Datatypes in SQLite Version 3)