sql - 为什么只能在 where 条件下以文本形式找到浮点值

Question

为什么没有引用参数就找不到行？

CREATE TABLE test (id integer, value real);
INSERT INTO test VALUES (1, 0.1);

不起作用:

SELECT * FROM test where value = 0.1;
 id | value
----+-------
(0 rows)

有效!

SELECT * FROM test where value = '0.1';
 id | value
----+-------
  1 |   0.1
(1 row)

Answer 1

你应该避免 inexact data types(like real) :

Inexact means that some values cannot be converted exactly to the internal format and are stored as approximations, so that storing and retrieving a value might show slight discrepancies. Managing these errors and how they propagate through calculations is the subject of an entire branch of mathematics and computer science and will not be discussed here, except for the following points:

• If you require exact storage and calculations (such as for monetary amounts), use the numeric type instead.

• If you want to do complicated calculations with these types for anything important, especially if you rely on certain behavior in boundary cases (infinity, underflow), you should evaluate the implementation carefully.

• Comparing two floating-point values for equality might not always work as expected.

如果您坚持使用实数，请确保两个参数具有相同的数据类型:

SELECT * FROM test where value = 0.1::real;

DBFiddle Demo