Added information about floating point numbers.
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Docs/manual.texi
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Docs/manual.texi
@ -44369,6 +44369,7 @@ the server runs, for example, in @code{safe_mysqld} or @code{mysql.server}.
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* Problems with alias:: Problems with @code{alias}
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* Deleting from related tables:: Deleting Rows from Related Tables
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* No matching rows:: Solving Problems with No Matching Rows
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* Problems with float:: Problems with floating point comparison
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@end menu
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@node Case sensitivity, Using DATE, Query Issues, Query Issues
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@ -44611,7 +44612,7 @@ id's per query if the @code{related_column} is an index. If the
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number of arguments in the @code{IN} clause.
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@node No matching rows, , Deleting from related tables, Query Issues
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@node No matching rows, Problems with float, Deleting from related tables, Query Issues
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@appendixsubsec Solving Problems with No Matching Rows
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@cindex no matching rows
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@ -44639,16 +44640,10 @@ the table that was last removed from the query.
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@item
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If you are comparing @code{FLOAT} or @code{DOUBLE} columns with numbers that
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have decimals, you can't use @code{=}! This problem is common in most
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computer languages because floating-point values are not exact values:
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@example
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mysql> SELECT * FROM table_name WHERE float_column=3.5;
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->
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mysql> SELECT * FROM table_name WHERE float_column between 3.45 and 3.55;
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@end example
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In most cases, changing the @code{FLOAT} to a @code{DOUBLE} will fix this!
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have decimals, you can't use @code{'='}. This problem is common in most
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computer languages because floating-point values are not exact values.
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In most cases, changing the @code{FLOAT} to a @code{DOUBLE} will fix this.
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@xref{Problems with float}.
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@item
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If you still can't figure out what's wrong, create a minimal test that can
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@ -44666,6 +44661,128 @@ shell> mysql test2 < query.sql
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Post the test file using @code{mysqlbug} to @email{mysql@@lists.mysql.com}.
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@end enumerate
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@node Problems with float, , No matching rows, Query Issues
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@appendixsubsec Problems with floating point comparison
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Floating point numbers cause confusion sometimes, because these numbers
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are not stored as exact values inside computer architecture. What one
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can see on the screen usually is not the exact value of the number.
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Field types @code{FLOAT} and @code{DECIMAL} are such.
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@example
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CREATE TABLE t1 (i int, d1 decimal(9,2), d2 decimal(9,2));
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INSERT INTO t1 values (1, 101.40, 21.40), (1, -80.00, 0.00), (2, 0.00, 0.00),
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(2, -13.20, 0.00), (2, 59.60, 46.40), (2, 30.40, 30.40), (3, 37.00, 7.40),
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(3, -29.60, 0.00), (4, 60.00, 15.40), (4, -10.60, 0.00), (4, -34.00, 0.00),
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(5, 33.00, 0.00), (5, -25.80, 0.00), (5, 0.00, 7.20), (6, 0.00, 0.00),
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(6, -51.40, 0.00);
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mysql> SELECT i, SUM(d1) AS a, SUM(d2) AS b FROM t1 GROUP BY i HAVING a <> b;
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+------+--------+-------+
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| i | a | b |
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+------+--------+-------+
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| 1 | 21.40 | 21.40 |
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| 2 | 76.80 | 76.80 |
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| 3 | 7.40 | 7.40 |
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| 4 | 15.40 | 15.40 |
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| 5 | 7.20 | 7.20 |
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| 6 | -51.40 | 0.00 |
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+------+--------+-------+
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@end example
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The result is correct. Although the first five records look like they
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shouldn't pass the comparison test, they may do so because the
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difference between the numbers show up around tenth decimal, or so
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depending on computer architecture.
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The problem cannot be solved by using ROUND() (or similar function),
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because the result is still a floating point number. Example:
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@example
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mysql> SELECT i, ROUND(SUM(d1), 2) AS a, ROUND(SUM(d2), 2) AS b FROM t1 GROUP BY i HAVING a <> b;
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+------+--------+-------+
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| i | a | b |
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+------+--------+-------+
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| 1 | 21.40 | 21.40 |
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| 2 | 76.80 | 76.80 |
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| 3 | 7.40 | 7.40 |
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| 4 | 15.40 | 15.40 |
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| 5 | 7.20 | 7.20 |
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| 6 | -51.40 | 0.00 |
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+------+--------+-------+
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@end example
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This is what the numbers in row 'a' look like:
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@example
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mysql> SELECT i, ROUND(SUM(d1), 2)*1.0000000000000000 AS a, ROUND(SUM(d2), 2) AS b FROM t1 GROUP BY i HAVING a <> b;
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+------+----------------------+-------+
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| i | a | b |
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+------+----------------------+-------+
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| 1 | 21.3999999999999986 | 21.40 |
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| 2 | 76.7999999999999972 | 76.80 |
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| 3 | 7.4000000000000004 | 7.40 |
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| 4 | 15.4000000000000004 | 15.40 |
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| 5 | 7.2000000000000002 | 7.20 |
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| 6 | -51.3999999999999986 | 0.00 |
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+------+----------------------+-------+
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@end example
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Depending on the computer architecture you may or may not see similar results.
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Each CPU may evaluate floating point numbers differently. For example in
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some machines you may get 'right' results by multiplaying both arguments
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with 1, an example follows.
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@strong{WARNING: NEVER TRUST THIS METHOD IN YOUR APPLICATION, THIS IS
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AN EXAMPLE OF A WRONG METHOD!!!}
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@example
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mysql> SELECT i, ROUND(SUM(d1), 2)*1 AS a, ROUND(SUM(d2), 2)*1 AS b FROM t1 GROUP
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BY i HAVING a <> b;
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+------+--------+------+
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| i | a | b |
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+------+--------+------+
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| 6 | -51.40 | 0.00 |
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+------+--------+------+
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@end example
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The reason why the above example seems to be working is that on the
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particular machine where the test was done, the CPU floating point
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arithmetics happens to round the numbers to same, but there is no
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rule that any CPU should do so, so it cannot be trusted.
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The correct way to do floating point number comparison is to first
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decide on what is the wanted tolerance between the numbers and then do
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the comparsion against the tolerance number. For example, if we agree on
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that floating point numbers should be regarded the same, if they are
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same with precision of one of ten thousand (0.0001), the comparsion
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should be done like this:
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@example
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mysql> SELECT i, SUM(d1) AS a, SUM(d2) AS b FROM t1 GROUP BY i HAVING ABS(a - b) > 0.0001;
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+------+--------+------+
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| i | a | b |
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+------+--------+------+
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| 6 | -51.40 | 0.00 |
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+------+--------+------+
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1 row in set (0.00 sec)
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@end example
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And vice versa, if we wanted to get rows where the numbers are the same,
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the test would be:
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@example
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mysql> SELECT i, SUM(d1) AS a, SUM(d2) AS b FROM t1 GROUP BY i HAVING ABS(a - b) < 0.0001;
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+------+-------+-------+
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| i | a | b |
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+------+-------+-------+
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| 1 | 21.40 | 21.40 |
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| 2 | 76.80 | 76.80 |
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| 3 | 7.40 | 7.40 |
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| 4 | 15.40 | 15.40 |
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| 5 | 7.20 | 7.20 |
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+------+-------+-------+
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@end example
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@node Table Definition Issues, , Query Issues, Problems
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@appendixsec Table Definition Related Issues
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