sqlite3比较不同类型的数据

SQLite3里比较表达式两端的数据类型不一样时会先做类型转化。以下例子很好的说明了转化规则：

--dbext: profile=sqlite

DROP TABLE IF EXISTS t1;
CREATE TABLE t1(
    a TEXT,      -- text affinity
    b NUMERIC,   -- numeric affinity
    c BLOB,      -- no affinity
    d            -- no affinity
);

-- Values will be stored as TEXT, INTEGER, TEXT, and INTEGER respectively
INSERT INTO t1 VALUES('500', '500', '500', 500);
SELECT typeof(a), typeof(b), typeof(c), typeof(d) FROM t1;
text|integer|text|integer

-- Because column "a" has text affinity, numeric values on the
-- right-hand side of the comparisons are converted to text before
-- the comparison occurs.
SELECT a < 40,   a < 60,   a < 600 FROM t1;
SELECT 40 > a,   60 > a,   600 > a FROM t1;
0|1|1

-- Text affinity is applied to the right-hand operands but since
-- they are already TEXT this is a no-op; no conversions occur.
SELECT a < '40', a < '60', a < '600' FROM t1;
0|1|1

-- Column "b" has numeric affinity and so numeric affinity is applied
-- to the operands on the right.  Since the operands are already numeric,
-- the application of affinity is a no-op; no conversions occur.  All
-- values are compared numerically.
SELECT b < 40,   b < 60,   b < 600 FROM t1;
0|0|1

-- Numeric affinity is applied to operands on the right, converting them
-- from text to integers.  Then a numeric comparison occurs.
SELECT b < '40', b < '60', b < '600' FROM t1;
0|0|1
SELECT '40' > b, '60' > b, '600' > b FROM t1;

-- No affinity conversions occur.  Right-hand side values all have
-- storage class INTEGER which are always less than the TEXT values
-- on the left.
SELECT c < 40,   c < 60,   c < 600 FROM t1;
0|0|0

-- No affinity conversions occur.  Values are compared as TEXT.
SELECT c < '40', c < '60', c < '600' FROM t1;
0|1|1

-- No affinity conversions occur.  Right-hand side values all have
-- storage class INTEGER which compare numerically with the INTEGER
-- values on the left.
SELECT d < 40,   d < 60,   d < 600 FROM t1;
0|0|1

-- No affinity conversions occur.  INTEGER values on the left are
-- always less than TEXT values on the right.
SELECT d < '40', d < '60', d < '600' FROM t1;
1|1|1

 

它和官方文档http://www.sqlite.org/datatype3.html 描述的有一定出入。所以最好的办法是对所有涉及不同数据类型的表达式某一段进行显示的数据类型转换，以消除歧义。上述例子中对a列可以用：

SELECT cast(a as integer) < 40,
       cast(a as integer) < 60,
       cast(a as integer) < 600
  FROM t1;

 消除SQLite类型转化时的模糊之处。