Add erdn sql command for SQL DDL export with multi-DBMS support

README.md

@@ -52,6 +52,7 @@ go build -o erdn ./cmd/erdn
 ```
 erdn render   <schema.erdn> [--out <file.svg>]
 erdn validate <schema.erdn>
+erdn sql      <schema.erdn> [--dbms <mysql|postgresql|mssql|oracle|sqlite>] [--out <file.sql>]
 ```
 
 ### `render`
@@ -75,6 +76,37 @@ erdn validate schema.erdn
 # OK
 ```
 
+### `sql`
+
+Generates SQL DDL (`CREATE TABLE`, indexes, and foreign key constraints) from the schema. Use `--dbms` to target a specific database engine (default: `mysql`).
+
+| DBMS | Flag value |
+|------|-----------|
+| MySQL | `mysql` |
+| PostgreSQL | `postgresql` |
+| Microsoft SQL Server | `mssql` |
+| Oracle Database | `oracle` |
+| SQLite | `sqlite` |
+
+```sh
+# Output written to schema.sql by default (MySQL)
+erdn sql schema.erdn
+
+# Target PostgreSQL
+erdn sql schema.erdn --dbms postgresql
+
+# Specify a custom output path
+erdn sql schema.erdn --dbms mssql --out migrations/001_init.sql
+```
+
+The generated SQL includes:
+
+- `CREATE TABLE` statements with DBMS-appropriate types and constraints.
+- `PRIMARY KEY` constraints.
+- `AUTO_INCREMENT` / `IDENTITY(1,1)` / `GENERATED ALWAYS AS IDENTITY` for auto-increment columns.
+- `CREATE INDEX` statements for `indexed` columns.
+- `ALTER TABLE … ADD CONSTRAINT … FOREIGN KEY` statements derived from `link` declarations (inline `FOREIGN KEY` for SQLite).
+
 ---
 
 ## Syntax

cmd/erdn/main.go

@@ -12,6 +12,7 @@ import (
 	"github.com/headercat/erdn-lang/internal/parser"
 	"github.com/headercat/erdn-lang/internal/render"
 	"github.com/headercat/erdn-lang/internal/semantic"
+	"github.com/headercat/erdn-lang/internal/sqlexport"
 )
 
 func main() {
@@ -25,6 +26,8 @@ func main() {
 		runRender(os.Args[2:])
 	case "validate":
 		runValidate(os.Args[2:])
+	case "sql":
+		runSQL(os.Args[2:])
 	default:
 		fmt.Fprintf(os.Stderr, "unknown subcommand: %s\n", os.Args[1])
 		printUsage()
@@ -33,11 +36,13 @@ func main() {
 }
 
 func printUsage() {
-	fmt.Fprintln(os.Stderr, `erdn - erdn-lang schema toolchain
+	fmt.Fprintf(os.Stderr, `erdn - erdn-lang schema toolchain
 
 Usage:
-  erdn render <schema.erdn> [--out <file>]
-  erdn validate <schema.erdn>`)
+  erdn render   <schema.erdn> [--out <file>]
+  erdn validate <schema.erdn>
+  erdn sql      <schema.erdn> [--dbms <%s>] [--out <file>]
+`, strings.Join(sqlexport.SupportedDBMS, "|"))
 }
 
 // loadAndValidate parses and semantically validates a schema file.
@@ -122,3 +127,42 @@ func runValidate(args []string) {
 	}
 	fmt.Println("OK")
 }
+
+func runSQL(args []string) {
+	fs := flag.NewFlagSet("sql", flag.ExitOnError)
+	dbmsFlag := fs.String("dbms", string(sqlexport.DBMSMySQL),
+		"target DBMS ("+strings.Join(sqlexport.SupportedDBMS, ", ")+")")
+	outFlag := fs.String("out", "", "output file path (default: <schema>.sql)")
+	if err := fs.Parse(args); err != nil {
+		os.Exit(1)
+	}
+	if fs.NArg() < 1 {
+		fmt.Fprintln(os.Stderr, "sql: missing schema file")
+		os.Exit(1)
+	}
+	if !sqlexport.ValidDBMS(*dbmsFlag) {
+		fmt.Fprintf(os.Stderr, "sql: unknown DBMS %q; supported: %s\n",
+			*dbmsFlag, strings.Join(sqlexport.SupportedDBMS, ", "))
+		os.Exit(1)
+	}
+
+	schemaFile := fs.Arg(0)
+	prog, err := loadAndValidate(schemaFile)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+
+	outPath := *outFlag
+	if outPath == "" {
+		base := strings.TrimSuffix(schemaFile, filepath.Ext(schemaFile))
+		outPath = base + ".sql"
+	}
+
+	sql := sqlexport.Generate(prog, sqlexport.DBMS(*dbmsFlag))
+	if err := os.WriteFile(outPath, []byte(sql), 0644); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	fmt.Printf("exported %s\n", outPath)
+}