Sql set values of one column equal to values of another column in the same table

Handling nulls in the target column

To customize your update and only modify rows where the target_column is NULL, the syntax slightly changes:

UPDATE table_name SET target_column = source_column WHERE target_column IS NULL;
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This essential tweak ensures the NULL gaps in the target_column are filled, leaving other existing data untouched.

Effective non-null updates using COALESCE

To guard against potentially overwriting valuable data in target_column, you could find the COALESCE function to be your next best friend:

UPDATE table_name SET target_column = COALESCE(target_column, source_column);
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COALESCE sticks to the first non-null value it finds. So, target_column only gets updated if it holds a NULL.

Crucial precautionary measures

Before you venture into updating:

• Ensure you have a backup of your table to circle back to in case of data mishaps.
• To maintain data integrity, check that source_column doesn't contain any NULL values if NULL in target_column is unacceptable.
• Consider wrapping your UPDATE commands within transactions for a managed execution:

START TRANSACTION;
UPDATE table_name SET target_column = source_column;
-- Verify the updates now, it's easier than explaining to your boss later
COMMIT; -- All good? Let's celebrate
ROLLBACK; -- Houston, we had a problem. But it's okay, we got backups.
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Dealing with complex update conditions

Raising the bar with update complexities:

• Conditional updates: Use the CASE statement or IF() function for small doses of complications:

UPDATE table_name SET target_column = 
CASE 
  WHEN some_condition THEN source_column 
  ELSE target_column 
END;
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• Foreign key constraints & triggers: Brush over any foreign key constraints or triggers that might be party-poopers to your update fiesta.
• Joint updates: When updates depend on another table, flex your SQL muscles with an UPDATE JOIN combo:

UPDATE a
SET a.target_column = b.source_column
FROM table_name a
JOIN another_table b ON a.join_key = b.join_key; --Joined at the hip
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Upholding data integrity

Data safety sure can keep one up at night:

• New table creation: As a safety net, craft a new table with tweaked values:

CREATE TABLE new_table AS
SELECT *, COALESCE(target_column, source_column) AS new_target_column
FROM table_name; --Voila, a new table is born.
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• Change auditing: For the meticulous, keep an alert eye on changes by noting down existing values before updating. Drop them into a temporary table or a sophisticated change tracking mechanism.