But COALESCE() intercepts the NULL value and replaces it with the string 'kitten'. Here NULLIF() checks if 'kitten' equals to 'kitten' and since it does, it returns NULL. Now let’s see the two together: SELECT COALESCE(NULLIF('kitten', 'kitten'), 'kitten') AS kitten If it wasn’t NULL, it returned that value: SELECT COALESCE('not kitten', 'kitten') AS kitten kitten | -| not kitten| For example: SELECT COALESCE(NULL, 'kitten') AS kittenīecause COALESCE replaced the NULL value with the string 'kitten'.
This operator detects NULL values and replaces them with a substitute.
Postgresql update how to#
How to use NULLIF() to turn an UPDATE row into a conditional one? By combining it with another operator, COALESCE(). But if you write this: SELECT NULLIF('kitten', 'not kitten') AS kitten In this case the string 'kitten' equals to 'kitten', so the result is NULL. If the two are identical, it returns NULL, otherwise returns the first value. Why? The NULLIF() operator takes the first parameter and compares it with the second one. Here’s a basic example: SELECT NULLIF('kitten', 'kitten') AS kitten This handy little tool turns any value into a NULL (as in PostgreSQL’s NULL, not JavaScript null) and allows to throw away unnecessary values. The heart of the solution is the NULLIF() operator. If you don’t have a value for any of the columns, you can simply pass an empty string and let PostgreSQL ignore it. You can actually make an update conditional. Good programmers don’t shovel data in their code. This is good enough to let you pass an exam, but a strict tech lead might spin you around by your tail for a few minutes for this. UPDATE users SET name = 'John Doe', email = phone = '12345678910', city = 'Whatever City' WHERE id = 3242 Thus you’re overwriting everything, but some fields with their original values, so they don’t actually change. The non-elegant solution of updating your data: get the user’s complete record, modify the fields that need to be modified in PHP, Node.js or whatever you’re using, then send this data to PostgreSQL, in a simple UPDATE query. It’s not what you’d do in a really serious project that needs high security, such as a national election’s database (albeit some companies might disagree with me here), but it’s more than adequate for your average office project. But this isn’t the preferred way of most programmers, as generating queries in PHP or Node.js is usually simpler and provides more control. The super-proper tight-ass method is to store all queries in SQL and call them as methods, using parameters. Granted, this isn’t the most SQL-ish solution in the world. In this article I’m assuming that you’re generating your PostgreSQL query as text, then sending it to the PostgreSQL server. But the user is changing only the email address, so there’s no need to update all the other values in the data row. There are many different fields:, name, email address, phone number, etc. Let’s say we are updating a user’s profile. What should you do if you only want your column to be updated if the supplied value is valid? It might be straightforward for some but I got stuck on this problem wondering what's going on so hopefully, it will help others.Conditional update in PostgreSQL (updated!) Instead, you must use all the tables in the FROM clause like this: UPDATE join_a_b JOIN b - Not valid since there is no ON clause JOIN b on b.id = join_a_b.b_id - Not valid since join_a_b is used here This means that basically, the following queries are not valid: UPDATE join_a_b Postgres wants a ON clause after the JOIN so you cannot only use where clauses.you cannot use the table you want to update to JOIN another one.To add something quite important to all the great answers above, when you want to update a join-table, you may have 2 problems: And there is no more need to JOIN table with itself (as it was in subquery). SET documents_taken_at = b.certificate_issued_at - we can reference joined table hereĪ.abiturient_id = b.id AND - JOIN ON clauseĪ.documents_taken_at::date < b.certificate_issued_at - Subquery WHEREĪs you can see, original subquery JOIN's ON clause have become one of WHERE conditions, which is conjucted by AND with others, which have been moved from subquery with no changes.
SET a.documents_taken_at = b.certificate_issued_at īecomes PostgreSQL-like in such a way UPDATE applications a WHERE ap.documents_taken_at::date < ab.certificate_issued_at So, we will increase application submit date to fit certificate issue date. Task: correct info, where abiturients (students about to leave secondary school) have submitted applications to university earlier, than they got school certificates (yes, they got certificates earlier, than they were issued (by certificate date specified). Let me explain a little more by my example.
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