1. 实现配置文件解析

2. 实现数据库连接

vendor/gorm.io/gorm/clause/select.go

@@ -0,0 +1,59 @@
+package clause
+
+// Select select attrs when querying, updating, creating
+type Select struct {
+	Distinct   bool
+	Columns    []Column
+	Expression Expression
+}
+
+func (s Select) Name() string {
+	return "SELECT"
+}
+
+func (s Select) Build(builder Builder) {
+	if len(s.Columns) > 0 {
+		if s.Distinct {
+			builder.WriteString("DISTINCT ")
+		}
+
+		for idx, column := range s.Columns {
+			if idx > 0 {
+				builder.WriteByte(',')
+			}
+			builder.WriteQuoted(column)
+		}
+	} else {
+		builder.WriteByte('*')
+	}
+}
+
+func (s Select) MergeClause(clause *Clause) {
+	if s.Expression != nil {
+		if s.Distinct {
+			if expr, ok := s.Expression.(Expr); ok {
+				expr.SQL = "DISTINCT " + expr.SQL
+				clause.Expression = expr
+				return
+			}
+		}
+
+		clause.Expression = s.Expression
+	} else {
+		clause.Expression = s
+	}
+}
+
+// CommaExpression represents a group of expressions separated by commas.
+type CommaExpression struct {
+	Exprs []Expression
+}
+
+func (comma CommaExpression) Build(builder Builder) {
+	for idx, expr := range comma.Exprs {
+		if idx > 0 {
+			_, _ = builder.WriteString(", ")
+		}
+		expr.Build(builder)
+	}
+}