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Document :exists difference

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Sean Corfield 2021-02-14 14:17:31 -08:00
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doc/differences-from-1-x.md
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@ -121,16 +121,28 @@ construction: `:constraint`, `:foreign-key`, `:index`, `:primary-key`,
You can now `SELECT` a function call more easily, using `[[...]]`. This was previously an error -- missing an alias -- but it was a commonly requested change, to avoid using `(sql/call ...)`:
```clojure
(sql/format {:select [:a [:b :c] [[:d :e]] [[:f :g] :h]]})
;; select a (column), b (aliased to c), d (fn call), f (fn call, aliased to h):
;;=> ["SELECT a, b AS c, D(e), F(g) AS h"]
user=> (sql/format {:select [:a [:b :c] [[:d :e]] [[:f :g] :h]]})
;; select a (column), b (aliased to c), d (fn call), f (fn call, aliased to h):
["SELECT a, b AS c, D(e), F(g) AS h"]
```
On a related note, `sql/call` has been removed because it should never be needed now: `[:foo ...]` should always be treated as a function call, consistently, avoiding the special cases in v1 that necessitated the explicit `sql/call` syntax.
The `:set` clause is dialect-dependent. In `:mysql`, it is ranked just before the `:where` clause. In all other dialects, it is ranked just before the `:from` clause. Accordingly, the `:set0` and `:set1` clauses are no longer supported (because they were workarounds in 1.x for this conflict).
HoneySQL 1.x implemented `:exists` as part of the DSL, which was incorrect:
it should have been a function, and in 2.x it is:
```clojure
;; 1.x: EXISTS should never have been implemented as SQL syntax: it's an operator!
;; (sq/format {:exists {:select [:a] :from [:foo]}})
;;=> ["EXISTS (SELECT a FROM foo)"]))
;; 2.x: select function call with an alias:
user=> (sql/format {:select [[[:exists {:select [:a] :from [:foo]}] :x]]})
["SELECT EXISTS (SELECT a FROM foo) AS x"]))
```
## Extensibility
The protocols and multimethods in 1.x have all gone away. The primary extension point is `honey.sql/register-clause!` which lets you specify the new clause (keyword), the formatter function for it, and the existing clause that it should be ranked before (`format` processes the DSL in clause order).