SQL Server 2012

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CREATE TABLE lamp
             (id integer IDENTITY
              PRIMARY KEY (id));
CREATE TABLE post
             (id integer IDENTITY
              PRIMARY KEY (id));
CREATE TABLE postlamp_t
             (post integer,
              lamp integer,
              lampindex integer IDENTITY,
              PRIMARY KEY (post,
                           lamp),
              FOREIGN KEY (post)
                          REFERENCES post
                                     (id),
              FOREIGN KEY (lamp)
                          REFERENCES lamp
                                     (id));
CREATE VIEW postlamp
AS
SELECT post,
       lamp,
       row_number() OVER (PARTITION BY post
                          ORDER BY lampindex) lampindex
       FROM postlamp_t;
INSERT INTO post
            DEFAULT VALUES;
INSERT INTO post
            DEFAULT VALUES;
            
INSERT INTO lamp
            DEFAULT VALUES;
INSERT INTO lamp
            DEFAULT VALUES;
INSERT INTO lamp
            DEFAULT VALUES;

INSERT INTO postlamp
            (post,
             lamp)
            VALUES (1,
                    1),
                   (2,
                    2),
                   (1,
                    3);

SELECT *
       FROM postlamp;


post
lamp
lampindex


1
1
1

1
3
2

2
2
1