1. 实现配置文件解析

2. 实现数据库连接

vendor/gorm.io/gorm/clause/joins.go

@@ -0,0 +1,47 @@
+package clause
+
+type JoinType string
+
+const (
+	CrossJoin JoinType = "CROSS"
+	InnerJoin JoinType = "INNER"
+	LeftJoin  JoinType = "LEFT"
+	RightJoin JoinType = "RIGHT"
+)
+
+// Join clause for from
+type Join struct {
+	Type       JoinType
+	Table      Table
+	ON         Where
+	Using      []string
+	Expression Expression
+}
+
+func (join Join) Build(builder Builder) {
+	if join.Expression != nil {
+		join.Expression.Build(builder)
+	} else {
+		if join.Type != "" {
+			builder.WriteString(string(join.Type))
+			builder.WriteByte(' ')
+		}
+
+		builder.WriteString("JOIN ")
+		builder.WriteQuoted(join.Table)
+
+		if len(join.ON.Exprs) > 0 {
+			builder.WriteString(" ON ")
+			join.ON.Build(builder)
+		} else if len(join.Using) > 0 {
+			builder.WriteString(" USING (")
+			for idx, c := range join.Using {
+				if idx > 0 {
+					builder.WriteByte(',')
+				}
+				builder.WriteQuoted(c)
+			}
+			builder.WriteByte(')')
+		}
+	}
+}