ALTER TABLE table_name MODIFY COLUMN column_name new_data_type;
时间: 2023-07-26 16:03:56 浏览: 55
是的,您可以使用 ALTER TABLE 语句中的 MODIFY COLUMN 子句来修改 MySQL 数据库表中的列定义,包括数据类型、长度、默认值、是否允许 NULL 值等属性。以下是使用 MODIFY COLUMN 子句来修改列数据类型和长度的示例:
```
ALTER TABLE table_name MODIFY COLUMN column_name new_data_type(new_length);
```
其中,new_data_type 是您想要更改列的数据类型,new_length 是您想要为列设置的新长度。例如,如果您想要将列的数据类型更改为 VARCHAR,并将其长度更改为 100,则可以使用以下语法:
```
ALTER TABLE table_name MODIFY COLUMN column_name VARCHAR(100);
```
请注意,更改列的数据类型和长度可能会导致数据丢失或转换错误,因此在执行此操作之前,请务必备份您的数据。
相关问题
sqlite数据库执行ALTER TABLE table_name MODIFY COLUMN column_name new_data_type; 提示near‘MODIFY’:syntax error
SQLite不支持使用`MODIFY COLUMN`关键字来修改列的数据类型。如果你想要修改列的数据类型,可以使用以下语法:
```
ALTER TABLE table_name RENAME TO temp_table_name;
CREATE TABLE table_name (
column1 datatype1 [ NULL | NOT NULL ],
column2 datatype2 [ NULL | NOT NULL ],
...
);
INSERT INTO table_name (column1, column2, ...)
SELECT column1, column2, ...
FROM temp_table_name;
DROP TABLE temp_table_name;
```
这个语法将会创建一个新表`table_name`,然后将原始表`table_name`中的数据插入到新表中,并且可以在新表中定义列的新数据类型。最后,删除临时表`temp_table_name`。注意,在进行这个操作之前请备份你的数据,以免数据丢失。
begin for r in (select a.TABLE_NAME,a.COLUMN_NAME,a.DATA_TYPE,a.DATA_LENGTH,b.comments from user_tab_columns a INNER JOIN user_col_comments b on a.table_name=b.table_name and a.column_name=b.column_name where data_type='NUMBER' and data_precision='20' and data_scale='2' and a.TABLE_NAME not IN('M_TOP_TEN_CREDIT_CUST','A_G15_ALL_RELATION_TXN','A_G15_TOP_TEN_CREDIT_CUST','V_GLF_CUST_INFO_NOREPEAT','A_INT_FEE','A_NONCREDIT_TXN_ALL') AND B.comments NOT IN ('12个月同类交易笔数','关联方期初持有本行的股数','关联方期末持有本行的股数','期初持有银行股数(股)','期末持有银行股数(股)指标值')) LOOP --增加临时新字段 execute immediate 'ALTER table "'|| r.TABLE_NAME ||'" ADD "'|| r.COLUMN_NAME ||'1" NUMBER(18,2)'; --复制内容 execute immediate 'Update "'|| r.TABLE_NAME ||'" Set "'|| r.COLUMN_NAME ||'1" = "'|| r.COLUMN_NAME ||'"'; --清空原有字段值 execute immediate 'Update "'|| r.TABLE_NAME ||'" Set "'|| r.COLUMN_NAME ||'" = null'; --修改原有的字段长度 execute immediate 'ALTER table "'|| r.TABLE_NAME ||'" MODIFY "'|| r.COLUMN_NAME ||'" NUMBER(18,2)'; --把值迁回原有字段 execute immediate 'Update "'|| r.TABLE_NAME ||'" Set "'|| r.COLUMN_NAME ||'" = "'|| r.COLUMN_NAME ||'1"'; --删除新增字段 execute immediate 'ALTER table "'|| r.TABLE_NAME ||'" drop column "'|| r.COLUMN_NAME ||'1"'; end loop; end ;改写成MySQL
在 MySQL 中,可以按照以下方式重写该代码:
```
BEGIN
DECLARE done INT DEFAULT 0;
DECLARE tableName VARCHAR(100);
DECLARE columnName VARCHAR(100);
DECLARE dataType VARCHAR(100);
DECLARE dataLength INT;
DECLARE comments VARCHAR(100);
DECLARE sql_stmt VARCHAR(1000);
DECLARE cur CURSOR FOR SELECT TABLE_NAME, COLUMN_NAME, DATA_TYPE, CHARACTER_MAXIMUM_LENGTH, COLUMN_COMMENT FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_SCHEMA = 'your_database_name' AND DATA_TYPE = 'decimal' AND NUMERIC_PRECISION = 20 AND NUMERIC_SCALE = 2 AND TABLE_NAME NOT IN ('M_TOP_TEN_CREDIT_CUST','A_G15_ALL_RELATION_TXN','A_G15_TOP_TEN_CREDIT_CUST','V_GLF_CUST_INFO_NOREPEAT','A_INT_FEE','A_NONCREDIT_TXN_ALL') AND COLUMN_COMMENT NOT IN ('12个月同类交易笔数','关联方期初持有本行的股数','关联方期末持有本行的股数','期初持有银行股数(股)','期末持有银行股数(股)指标值');
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = 1;
OPEN cur;
read_loop: LOOP
FETCH cur INTO tableName, columnName, dataType, dataLength, comments;
IF done THEN
LEAVE read_loop;
END IF;
SET sql_stmt = CONCAT('ALTER TABLE `', tableName, '` MODIFY COLUMN `', columnName, '` DECIMAL(18,2)');
PREPARE stmt FROM sql_stmt;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
SET sql_stmt = CONCAT('ALTER TABLE `', tableName, '` ADD COLUMN `', columnName, '1` DECIMAL(18,2)');
PREPARE stmt FROM sql_stmt;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
SET sql_stmt = CONCAT('UPDATE `', tableName, '` SET `', columnName, '1` = `', columnName, '`');
PREPARE stmt FROM sql_stmt;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
SET sql_stmt = CONCAT('UPDATE `', tableName, '` SET `', columnName, '` = NULL');
PREPARE stmt FROM sql_stmt;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
SET sql_stmt = CONCAT('UPDATE `', tableName, '` SET `', columnName, '` = `', columnName, '1`');
PREPARE stmt FROM sql_stmt;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
SET sql_stmt = CONCAT('ALTER TABLE `', tableName, '` DROP COLUMN `', columnName, '1`');
PREPARE stmt FROM sql_stmt;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
END LOOP;
CLOSE cur;
END;
```
需要注意的是,MySQL 中 DECIMAL 数据类型的定义方式与 Oracle 中的 NUMBER 数据类型不同。在 MySQL 中,DECIMAL(18,2) 表示该字段的总长度为 18 位,小数位为 2 位。此外,MySQL 中需要使用反引号来引用表名和字段名。