By default, MySQL is forgiving of illegal or improper data values and coerces them to legal values for data entry. However, you can change the server SQL mode to select more traditional treatment of bad values such that the server rejects them and aborts the statement in which they occur. See Section 5.1.7, “Server SQL Modes”.
This section describes the default (forgiving) behavior of MySQL, as well as the strict SQL mode and how it differs.
If you are not using strict mode, then whenever you insert an
“incorrect” value into a column, such as a
NULL into a NOT NULL
column or a too-large numeric value into a numeric column,
MySQL sets the column to the “best possible
value” instead of producing an error: The following
rules describe in more detail how this works:
If you try to store an out of range value into a numeric column, MySQL Server instead stores zero, the smallest possible value, or the largest possible value, whichever is closest to the invalid value.
For strings, MySQL stores either the empty string or as much of the string as can be stored in the column.
If you try to store a string that doesn't start with a number into a numeric column, MySQL Server stores 0.
Invalid values for
ENUMandSETcolumns are handled as described in Section 1.8.6.3, “ENUMandSETConstraints”.MySQL enables you to store certain incorrect date values into
DATEandDATETIMEcolumns (such as'2000-02-31'or'2000-02-00'). The idea is that it is not the job of the SQL server to validate dates. If MySQL can store a date value and retrieve exactly the same value, MySQL stores it as given. If the date is totally wrong (outside the server's ability to store it), the special “zero” date value'0000-00-00'is stored in the column instead.If you try to store
NULLinto a column that doesn't takeNULLvalues, an error occurs for single-rowINSERTstatements. For multiple-rowINSERTstatements or forINSERT INTO ... SELECTstatements, MySQL Server stores the implicit default value for the column data type. In general, this is0for numeric types, the empty string ('') for string types, and the “zero” value for date and time types. Implicit default values are discussed in Section 10.1.4, “Data Type Default Values”.If an
INSERTstatement specifies no value for a column, MySQL inserts its default value if the column definition includes an explicitDEFAULTclause. If the definition has no suchDEFAULTclause, MySQL inserts the implicit default value for the column data type.
The reason for using the preceding rules in nonstrict mode is that we can't check these conditions until the statement has begun executing. We can't just roll back if we encounter a problem after updating a few rows, because the storage engine may not support rollback. The option of terminating the statement is not that good; in this case, the update would be “half done,” which is probably the worst possible scenario. In this case, it is better to “do the best you can” and then continue as if nothing happened.
In MySQL 5.0.2 and up, you can select stricter treatment of
input values by using the
STRICT_TRANS_TABLES or
STRICT_ALL_TABLES SQL modes:
SET sql_mode = 'STRICT_TRANS_TABLES'; SET sql_mode = 'STRICT_ALL_TABLES';
STRICT_TRANS_TABLES enables
strict mode for transactional storage engines, and also to
some extent for nontransactional engines. It works like this:
For transactional storage engines, bad data values occurring anywhere in a statement cause the statement to abort and roll back.
For nontransactional storage engines, a statement aborts if the error occurs in the first row to be inserted or updated. (When the error occurs in the first row, the statement can be aborted to leave the table unchanged, just as for a transactional table.) Errors in rows after the first do not abort the statement, because the table has already been changed by the first row. Instead, bad data values are adjusted and result in warnings rather than errors. In other words, with
STRICT_TRANS_TABLES, a wrong value causes MySQL to roll back all updates done so far, if that can be done without changing the table. But once the table has been changed, further errors result in adjustments and warnings.
For even stricter checking, enable
STRICT_ALL_TABLES. This is
the same as
STRICT_TRANS_TABLES except
that for nontransactional storage engines, errors abort the
statement even for bad data in rows following the first row.
This means that if an error occurs partway through a
multiple-row insert or update for a nontransactional table, a
partial update results. Earlier rows are inserted or updated,
but those from the point of the error on are not. To avoid
this for nontransactional tables, either use single-row
statements or else use
STRICT_TRANS_TABLES if
conversion warnings rather than errors are acceptable. To
avoid problems in the first place, do not use MySQL to check
column content. It is safest (and often faster) to let the
application ensure that it passes only legal values to the
database.
With either of the strict mode options, you can cause errors
to be treated as warnings by using
INSERT
IGNORE or UPDATE IGNORE rather
than INSERT or
UPDATE without
IGNORE.