如果添加主键约束

ALTER TABLE <TABLE NAME> ADD CONSTRAINT <CONSTRAINT NAME> PRIMARY KEY <COLUMNNAME>  

例如:

ALTER TABLE DEPT ADD CONSTRAINT PK_DEPT PRIMARY KEY (DEPTNO)

试着使用下面的代码:

ALTER TABLE `table name` 
    CHANGE COLUMN `column name` `column name` datatype NOT NULL, 
    ADD PRIMARY KEY (`column name`) ;

Necromancing。 以防有人有和我一样好的图式…… 以下是正确的做法:

在本例中，表名为dbo。T_SYS_Language_Forms，列名是LANG_UID

-- First, chech if the table exists...
IF 0 < (
    SELECT COUNT(*) FROM INFORMATION_SCHEMA.TABLES 
    WHERE TABLE_TYPE = 'BASE TABLE'
    AND TABLE_SCHEMA = 'dbo'
    AND TABLE_NAME = 'T_SYS_Language_Forms'
)
BEGIN
    -- Check for NULL values in the primary-key column
    IF 0 = (SELECT COUNT(*) FROM T_SYS_Language_Forms WHERE LANG_UID IS NULL)
    BEGIN
        ALTER TABLE T_SYS_Language_Forms ALTER COLUMN LANG_UID uniqueidentifier NOT NULL 

        -- No, don't drop, FK references might already exist...
        -- Drop PK if exists (it is very possible it does not have the name you think it has...)
        -- ALTER TABLE T_SYS_Language_Forms DROP CONSTRAINT pk_constraint_name 
        --DECLARE @pkDropCommand nvarchar(1000) 
        --SET @pkDropCommand = N'ALTER TABLE T_SYS_Language_Forms DROP CONSTRAINT ' + QUOTENAME((SELECT CONSTRAINT_NAME FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS 
        --WHERE CONSTRAINT_TYPE = 'PRIMARY KEY' 
        --AND TABLE_SCHEMA = 'dbo' 
        --AND TABLE_NAME = 'T_SYS_Language_Forms' 
        ----AND CONSTRAINT_NAME = 'PK_T_SYS_Language_Forms' 
        --))
        ---- PRINT @pkDropCommand 
        --EXECUTE(@pkDropCommand) 
        -- Instead do
        -- EXEC sp_rename 'dbo.T_SYS_Language_Forms.PK_T_SYS_Language_Forms1234565', 'PK_T_SYS_Language_Forms';

        -- Check if they keys are unique (it is very possible they might not be)        
        IF 1 >= (SELECT TOP 1 COUNT(*) AS cnt FROM T_SYS_Language_Forms GROUP BY LANG_UID ORDER BY cnt DESC)
        BEGIN

            -- If no Primary key for this table
            IF 0 =  
            (
                SELECT COUNT(*) FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS 
                WHERE CONSTRAINT_TYPE = 'PRIMARY KEY' 
                AND TABLE_SCHEMA = 'dbo' 
                AND TABLE_NAME = 'T_SYS_Language_Forms' 
                -- AND CONSTRAINT_NAME = 'PK_T_SYS_Language_Forms' 
            )
                ALTER TABLE T_SYS_Language_Forms ADD CONSTRAINT PK_T_SYS_Language_Forms PRIMARY KEY CLUSTERED (LANG_UID ASC)
            ;

        END -- End uniqueness check
        ELSE
            PRINT 'FSCK, this column has duplicate keys, and can thus not be changed to primary key...' 
    END -- End NULL check
    ELSE
        PRINT 'FSCK, need to figure out how to update NULL value(s)...' 
END 

如果添加主键约束

ALTER TABLE <TABLE NAME> ADD CONSTRAINT <CONSTRAINT NAME> PRIMARY KEY <COLUMNNAME>  

例如:

ALTER TABLE DEPT ADD CONSTRAINT PK_DEPT PRIMARY KEY (DEPTNO)

在您的表中已经有一个主键。不能只添加主键，否则会引起错误。因为sql表只有一个主键。

首先，您必须删除旧的主键。

MySQL:

ALTER TABLE Persion
DROP PRIMARY KEY;

SQL Server / Oracle / MS访问:

ALTER TABLE Persion
DROP CONSTRAINT 'constraint name';

您必须在表中找到约束名称。如果你有 在创建表时给定约束名，可以轻松使用 约束名(例如:PK_Persion)。

第二，添加主键。

MySQL / SQL Server / Oracle / MS访问:

ALTER TABLE Persion ADD PRIMARY KEY (PersionId,Pname,PMID);

或者下面那个更好的

ALTER TABLE Persion ADD CONSTRAINT PK_Persion PRIMARY KEY (PersionId,Pname,PMID);

这可以由开发人员设置约束名称。这样更容易维护这个表。

把所有的答案看完后，我有点糊涂了。所以我研究了一些文件，找出每一个细节。希望这个答案可以帮助其他SQL初学者。

参考:https://www.w3schools.com/sql/sql_primarykey.asp

请尝尝这个

ALTER TABLE TABLE_NAME DROP INDEX `PRIMARY`, ADD PRIMARY KEY (COLUMN1, COLUMN2,..);