fix(mysql): Handle simplified CASE statements (#2852)

MySQL and PostgreSQL now have the same behavior, generating a nullable string parameter. That behavior isn't right, as we'd expect the parameter to be a boolean. This can be fixed by using a type case or the new annotations in #2800.

Fixes #2847

Author: kyleconroy

internal/endtoend/testdata/case_value_param/issue.md

@@ -0,0 +1 @@
+https://github.com/sqlc-dev/sqlc/issues/2847

internal/endtoend/testdata/case_value_param/mysql/go/db.go

@@ -0,0 +1,3 @@
+-- name: Update :exec
+UPDATE testing
+SET value = CASE ? WHEN true THEN 'Hello' WHEN false THEN 'Goodbye' ELSE value END;

internal/endtoend/testdata/case_value_param/mysql/go/models.go

@@ -0,0 +1,4 @@
+CREATE TABLE testing (
+  id    int PRIMARY KEY,
+  value text
+);

internal/endtoend/testdata/case_value_param/mysql/go/query.sql.go

@@ -0,0 +1,9 @@
+version: "2"
+sql:
+  - engine: "mysql"
+    schema: "schema.sql"
+    queries: "query.sql"
+    gen:
+      go:
+        package: "querytest"
+        out: "go"