在PostgresQL中是:

ALTER TABLE tableName ALTER COLUMN columnName DROP NOT NULL;

假设SQL Server(基于你之前的问题):

ALTER TABLE Merchant_Pending_Functions ALTER COLUMN NumberOfLocations INT NULL

将INT替换为实际的数据类型。

在PostgresQL中是:

ALTER TABLE tableName ALTER COLUMN columnName DROP NOT NULL;

确保您添加了要修改的列的data_type。

ALTER TABLE TABLE_NAME MODIFY COLUMN_NAME DATA_TYPE NULL;

SQLite

ALTER TABLE命令有点特殊。不可能修改列。你必须创建一个新列，迁移数据，然后删除该列:

-- 1. First rename
ALTER TABLE
    Merchant_Pending_Functions
RENAME COLUMN
    NumberOfLocations
TO
   NumberOfLocations_old

-- 2. Create new column
ALTER TABLE
    Merchant_Pending_Functions
ADD COLUMN
    NumberOfLocations INT NULL

-- 3. Migrate data - you need to write code for that
-- 4. Drop the old column
ALTER TABLE
    Merchant_Pending_Functions
DROP COLUMN
    NumberOfLocations_old

正如其他人所观察到的，该命令的精确语法在不同类型的DBMS中有所不同。你使用的语法在Oracle中工作:

SQL> desc MACAddresses
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 COMPUTER                                           NUMBER
 MACADDRESS                                         VARCHAR2(12)
 CORRECTED_MACADDRESS                      NOT NULL VARCHAR2(17)

SQL> alter table MACAddresses
  2       modify corrected_MACAddress null
  3  /

Table altered.

SQL> desc MACAddresses
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 COMPUTER                                           NUMBER
 MACADDRESS                                         VARCHAR2(12)
 CORRECTED_MACADDRESS                               VARCHAR2(17)

SQL>