func (b *Builder) Where(cond Cond) *Builder {
	b.cond = b.cond.And(cond)
	return b
}
Where sets where SQL
package builder
type optype byte
const (
	condType   optype = iota  
	selectType                
	insertType                
	updateType                
	deleteType                
)
type join struct {
	joinType  string
	joinTable string
	joinCond  Cond
}
type Builder struct {
	optype
	tableName string
	cond      Cond
	selects   []string
	joins     []join
	inserts   Eq
	updates   []Eq
}
func Select(cols ...string) *Builder {
	builder := &Builder{cond: NewCond()}
	return builder.Select(cols...)
}
func Insert(eq Eq) *Builder {
	builder := &Builder{cond: NewCond()}
	return builder.Insert(eq)
}
func Update(updates ...Eq) *Builder {
	builder := &Builder{cond: NewCond()}
	return builder.Update(updates...)
}
func Delete(conds ...Cond) *Builder {
	builder := &Builder{cond: NewCond()}
	return builder.Delete(conds...)
}