CREATE TABLE t1 (
  i     INT DEFAULT -1,
  c     VARCHAR(10) DEFAULT '',
  price DOUBLE(16,2) DEFAULT 0.00
);

The default value specified in a DEFAULT clause can be a literal constant or an expression. DEFAULT子句中指定的默认值可以是文字常量或表达式。With one exception, enclose expression default values within parentheses to distinguish them from literal constant default values. 除了一个例外，将表达式默认值括在括号内，以区别于文本常量默认值。Examples:例如：

CREATE TABLE t1 (
  -- literal defaults
  i INT         DEFAULT 0,
  c VARCHAR(10) DEFAULT '',
  -- expression defaults
  f FLOAT       DEFAULT (RAND() * RAND()),
  b BINARY(16)  DEFAULT (UUID_TO_BIN(UUID())),
  d DATE        DEFAULT (CURRENT_DATE + INTERVAL 1 YEAR),
  p POINT       DEFAULT (Point(0,0)),
  j JSON        DEFAULT (JSON_ARRAY())
);

The exception is that, for TIMESTAMP and DATETIME columns, you can specify the CURRENT_TIMESTAMP function as the default, without enclosing parentheses. 例外情况是，对于TIMESTAMP和DATETIME列，您可以将CURRENT_TIMESTAMP函数指定为默认值，而不使用括号。See Section 11.2.5, “Automatic Initialization and Updating for TIMESTAMP and DATETIME”.请参阅第11.2.5节，“时间戳和日期时间的自动初始化和更新”。

The BLOB, TEXT, GEOMETRY, and JSON data types can be assigned a default value only if the value is written as an expression, even if the expression value is a literal:BLOB、TEXT、GEOMETRY和JSON数据类型只能在值作为表达式写入时指定默认值，即使表达式值是文本：

Expression default values must adhere to the following rules. 表达式默认值必须遵循以下规则。An error occurs if an expression contains disallowed constructs.如果表达式包含不允许的构造，则会发生错误。

For CREATE TABLE ... LIKE and CREATE TABLE ... SELECT, the destination table preserves expression default values from the original table.对于CREATE TABLE ... LIKE语句和CREATE TABLE ... SELECT语句，目标表保留原始表中的表达式默认值。

If an expression default value refers to a nondeterministic function, any statement that causes the expression to be evaluated is unsafe for statement-based replication. 如果表达式默认值引用不确定函数，则任何导致表达式求值的语句对于基于语句的复制都是不安全的。This includes statements such as INSERT and UPDATE. 这包括INSERT语句和UPDATE等语句。In this situation, if binary logging is disabled, the statement is executed as normal. 在这种情况下，如果禁用二进制日志记录，则语句将正常执行。If binary logging is enabled and binlog_format is set to STATEMENT, the statement is logged and executed but a warning message is written to the error log, because replication slaves might diverge. 如果启用了二进制日志记录，并且binlog_format设置为STATEMENT，则会记录并执行该语句，但会向错误日志中写入警告消息，因为复制从属服务器可能会发生分歧。When binlog_format is set to MIXED or ROW, the statement is executed as normal.当binlog_format设置为MIXED或ROW时，语句将正常执行。

When inserting a new row, the default value for a column with an expression default can be inserted either by omitting the column name or by specifying the column as DEFAULT (just as for columns with literal defaults):插入新行时，可以通过省略列名或将列指定为DEFAULT来插入表达式为默认值的列的默认值（与使用文字默认值的列相同）：

mysql> CREATE TABLE t4 (uid BINARY(16) DEFAULT (UUID_TO_BIN(UUID())));
mysql> INSERT INTO t4 () VALUES();
mysql> INSERT INTO t4 () VALUES(DEFAULT);
mysql> SELECT BIN_TO_UUID(uid) AS uid FROM t4;
+--------------------------------------+
| uid                                  |
+--------------------------------------+
| f1109174-94c9-11e8-971d-3bf1095aa633 |
| f110cf9a-94c9-11e8-971d-3bf1095aa633 |
+--------------------------------------+

However, the use of DEFAULT(col_name) to specify the default value for a named column is permitted only for columns that have a literal default value, not for columns that have an expression default value.但是，仅允许对具有文字默认值的列使用DEFAULT(col_name)来指定命名列的默认值，而不允许对具有表达式默认值的列使用它。

Not all storage engines permit expression default values. 并非所有存储引擎都允许表达式默认值。For those that do not, an ER_UNSUPPORTED_ACTION_ON_DEFAULT_VAL_GENERATED error occurs.对于那些不支持的，会发生一个ER_UNSUPPORTED_ACTION_ON_DEFAULT_VAL_GENERATED错误。

If a default value evaluates to a data type that differs from the declared column type, implicit coercion to the declared type occurs according to the usual MySQL type-conversion rules. 如果默认值的计算结果是与声明的列类型不同的数据类型，则会根据通常的MySQL类型转换规则对声明的类型进行隐式强制。See Section 12.3, “Type Conversion in Expression Evaluation”.请参阅第12.3节“表达式求值中的类型转换”。

With one exception, the default value specified in a DEFAULT clause must be a literal constant; it cannot be a function or an expression. 除了一个例外，DEFAULT子句中指定的默认值必须是一个文本常量；它不能是函数或表达式。This means, for example, that you cannot set the default for a date column to be the value of a function such as NOW() or CURRENT_DATE. 例如，这意味着您不能将日期列的默认值设置为函数的值，例如NOW()或CURRENT_DATE。The exception is that, for TIMESTAMP and DATETIME columns, you can specify CURRENT_TIMESTAMP as the default. 例外情况是，对于TIMESTAMP和DATETIME列，可以将CURRENT_TIMESTAMP指定为默认值。See Section 11.2.5, “Automatic Initialization and Updating for TIMESTAMP and DATETIME”.请参阅第11.2.5节，“时间戳和日期时间的自动初始化和更新”。

The BLOB, TEXT, GEOMETRY, and JSON data types cannot be assigned a default value.不能为BLOB、TEXT、GEOMETRY和JSON数据类型分配默认值。

If a default value evaluates to a data type that differs from the declared column type, implicit coercion to the declared type occurs according to the usual MySQL type-conversion rules. 如果默认值的计算结果是与声明的列类型不同的数据类型，则会根据通常的MySQL类型转换规则对声明的类型进行隐式强制。See Section 12.3, “Type Conversion in Expression Evaluation”.请参阅第12.3节，“表达式求值中的类型转换”。

If a data type specification includes no explicit DEFAULT value, MySQL determines the default value as follows:如果数据类型规范不包含显式的DEFAULT值，MySQL将按如下方式确定默认值：

If the column can take NULL as a value, the column is defined with an explicit DEFAULT NULL clause.如果该列可以将NULL作为值，则该列将使用显式的DEFAULT NULL子句定义。

If the column cannot take NULL as a value, MySQL defines the column with no explicit DEFAULT clause.如果列不能将NULL作为值，MySQL将定义没有显式DEFAULT子句的列。

For data entry into a NOT NULL column that has no explicit DEFAULT clause, if an INSERT or REPLACE statement includes no value for the column, or an UPDATE statement sets the column to NULL, MySQL handles the column according to the SQL mode in effect at the time:对于没有显式DEFAULT子句的NOT NULL列中的数据输入，如果INSERT或REPLACE语句不包含该列的值，或者UPDATE语句将该列设置为NULL，则MySQL将根据当时有效的SQL模式处理该列：

Suppose that a table t is defined as follows:假设表t的定义如下：

CREATE TABLE t (i INT NOT NULL);

In this case, i has no explicit default, so in strict mode each of the following statements produce an error and no row is inserted. 在本例中，i没有显式的默认值，因此在严格模式下，下面的每个语句都会产生一个错误，并且不会插入任何行。When not using strict mode, only the third statement produces an error; the implicit default is inserted for the first two statements, but the third fails because DEFAULT(i) cannot produce a value:当不使用严格模式时，只有第三条语句产生错误；为前两个语句插入隐式默认值，但第三个语句失败，因为DEFAULT(i)不能生成值：

INSERT INTO t VALUES();
INSERT INTO t VALUES(DEFAULT);
INSERT INTO t VALUES(DEFAULT(i));

See Section 5.1.11, “Server SQL Modes”.请参阅第5.1.11节，“服务器SQL模式”。

For a given table, the SHOW CREATE TABLE statement displays which columns have an explicit DEFAULT clause.对于给定的表，SHOW CREATE TABLE语句显示哪些列具有显式DEFAULT子句。

Implicit defaults are defined as follows:隐式默认值定义如下：