func NewCase(expr sql.Expression, branches []CaseBranch, elseExpr sql.Expression) *Case {
	return &Case{expr, branches, elseExpr}
}
NewCase returns an new Case expression.
package expression
import (
	"bytes"
	"github.com/src-d/go-mysql-server/sql"
)
type CaseBranch struct {
	Cond  sql.Expression
	Value sql.Expression
}
type Case struct {
	Expr     sql.Expression
	Branches []CaseBranch
	Else     sql.Expression
}