I'll definitely keep this in my back pocket. This allows us to keep the correlated subquery, and all of it's power. I liked the "array" answer so much I wanted to add a complete example for PostgreSQL 9.3, using the generic information schema. The CASE statement goes through various conditions and returns values as and when the first condition is met (like an IF-THEN-ELSE statement in. It is a kind of control statement which forms the cell of programming languages as they control the execution of other sets of statements. So, that is in fact a handy solution (in 9.1 at least): make a function to extract your highest priority row by doing the limit inside the function.īut functions have the drawback that the query plan will not show what is going on inside them and I believe it will always choose a nested loop join, even when that might not be best. CASE statement in MySQL is a way of handling the if/else logic. Left join (select * from product_special ps where ps.id = p.id order by priority desc limit 1) on trueĮven though the definition of the function is precisely that. Left join highestPriorityProductSpecial(p.id) on true PostgreSQL made it possible to do this by making kind of an exception - you can pass parameters like that if the expression is a simple function call but not strictly speaking an embedded SELECT. But it turned out to be useful for all kinds of situations where you want to avoid another level of subquery nesting or moving things from the FROM clause to the SELECT clause.
#Mysql case select multiple columns how to
Basically, it was their solution for how to pass parameters (which tend to be references to columns external to the current table expression) to functions called as table expressions in the FROM clause. Note that the "cross apply" mechanism from SQL Server would solve this, but it isn't available in PostgreSQL.
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