SQL更新唯一键值对

Question

I have a table that has a unique non-clustered constraint on two columns (let's call them key and value )

CREATE TABLE table (
  id varchar(50), key varchar(64) NOT NULL, value varchar(64) NOT NULL, ...
  CONSTRAINT pk_table_id PRIMARY KEY CLUSTERED (id ASC) WITH (...) ON [PRIMARY],
  CONSTRAINT pk_key_value UNIQUE NONCLUSTERED ( key ASC, value ASC) WITH (...) ON PRIMARY
) ON [PRIMARY]

Now I need to write a safe update script that updates all rows that have key='oldKey' , where safe update script means it has to be possible to be executed multiple times, without throwing an exception.

What I had at the beginning was:

UPDATE table
SET key = 'newKey'
WHERE key = 'oldKey'

which worked at the first execution (due to the constraint).

Let's say I have a row with key='oldKey' and value='b' , that got updated to key='newKey' and value='b' . Now two new rows are inserted, one with key='oldKey' and value='b' and the other with key='oldKey' and value='c' , and the update script is run again. Because the key is updated to 'newKey' and the key-value pair newKey, b already exists, an exception is thrown.

What I'm trying to achieve now, is to make an update statement that updates all rows that have key='oldKey' AND no key-value pair key='newKey' value='b' already exists, otherwise do nothing.

this is what I tried so far:

-- 1st try, nothing is updated
IF NOT EXISTS (SELECT * FROM table WHERE key='newKey' AND value IN (SELECT value FROM table WHERE key='oldKey'))
BEGIN
  UPDATE table
  SET key = 'newKey'
  WHERE key = 'oldKey'
END

-- 2nd try, exception thrown
UPDATE table
SET key = 'newKey'
WHERE key = 'oldKey' AND
  (SELECT COUTN(*) FROM table WHERE key='newKey' AND value IN (SELECT value FROM table WHERE key='oldKey')) > 0

-- 3rd try, nothing is updated
UPDATE table
SET key = 'newKey'
WHERE NOT EXISTS (SELECT * FROM table WHERE key='newKey' AND value IN (SELECT value FROM table WHERE key='oldKey'))

Answer 1

Try this:

UPDATE t1
SET key = 'newKey'
from table t1
WHERE key = 'oldKey'
and NOT EXISTS (SELECT * FROM table t2 WHERE t2.key='newKey' AND t1.value=t2.value)

Answer 2

TRY THIS :

UPDATE t SET key = 'newKey' 
FROM yourtable t
LEFT JOIN yourtable t1 ON t1.value = t.value
    AND t1.key = 'newKey'
WHERE t.key = 'oldKey'
    AND t1.value IS NULL