用例：有一段sql语句，我们需要从中截取出所有字段部分，以便进行后续的类型推断，请给出此解析方法。
想来很简单吧，因为 sql 中的字段列表，使用方式有限，比如 a as b, a, a b...
1. 解题思路
如果不想做复杂处理，最容易想到的，就是直接用某个特征做分割即可。比如，先截取出 字段列表部分，然后再用逗号',' 分割，就可以得到一个个的字段了。然后再要细分，其实只需要用 as 进行分割就可以了。
看起来好像可行，但是存在许多漏洞，首先，这里面有太多的假设：各种截取部分要求必须符合要求，必须没有多余的逗号，必须要有as 等等。这明显不符合要求了。
其二，我们可以换一种转换方式。比如先截取到field部分，然后先以 as 分割，再以逗号分割，然后取最后一个词作为field。
看起来好像更差了，截取到哪里已经完全不知道了。即原文已经被破坏殆尽，而且同样要求要有 as 转换标签，而且对于函数觊觎有 as 的场景，就完全错误了。
其三，最好还是自行一个个单词地解析，field 字段无外乎几种情况，1. 普通字段如 select a; 2. 带as的普通字段如 select a as b; 3. 带函数的字段如 select coalesce(a, b); 4. 带函数且带as的字段如 select coalesce(a, b) ab; 5. 函数内带as的字段如 select cast(a as string) b; ... 我们只需依次枚举对应的情况，就可以将字段解析出来了。
看起来是个不错的想法。但是具体实现如何？
2. 具体解析实现
主要分两个部分，1. 需要定义一个解析后的结果数据结构，以便清晰描述字段信息； 2. 分词解析sql并以结构体返回；
我们先来看看整个算法核心：
/** * 功能描述: 简单sql字段解析器 * * 样例如1: * select COALESCE(t1.xno, t2.xno, t3.xno) as xno, * case when t1.no is not null then 1 else null end as xxk001, * case when t2.no is not null then 1 else null end as xxk200, * case when t3.xno is not null then 1 else null end as xx3200 * from xxk001 t1 * full join xxkj100 t2 on t1.xno = t2.xno * full join xxkj200 t3 on t1.xno = t3.xno; * * 样例如2: * select cast(a as string) as b from ccc; * * 样例如3: * with a as(select cus,x1 from b1), b as (select cus,x2 from b2) * select a.cus as a_cus from a join b on a.cus=b.cus where xxx; * * 样例如4: * select a.xno,b.xx from a_tb as a join b_tb as b on a.id = b.id * * 样例如5: * select cast \t(a as string) a_str, cc (a as double) a_double from x * */ public class SimpleSqlFieldParser { /** * 解析一段次标签sql 中的字段列表 * * @param sql 原始sql, 需如 select xx from xxx join ... 格式 * @return 字段列表 */ public static List<SelectFieldClauseDescriptor> parse(String sql) { String columnPart = adaptFieldPartSql(sql); int deep = 0; List<StringBuilder> fieldTokenSwap = new ArrayList<>(); StringBuilder currentTokenBuilder = new StringBuilder(); List<SelectFieldClauseDescriptor> fieldList = new ArrayList<>(); fieldTokenSwap.add(currentTokenBuilder); int len = columnPart.length(); char[] columnPartChars = columnPart.toCharArray(); for(int i = 0; i < len; i++) { // 空格忽略，换行忽略，tab忽略 // 字符串相接 // 左(号入栈，++deep； // 右)号出栈，--deep； // deep>0 忽略所有其他直接拼接 // as 则取下一个值为fieldName // case 则直接取到end为止； //,号则重置token，构建结果集 char currentChar = columnPartChars[i]; switch (currentChar) { case '(': ++deep; currentTokenBuilder.append(currentChar); break; case ')': --deep; currentTokenBuilder.append(currentChar); break; case ',': if(deep == 0) { addNewField(fieldList, fieldTokenSwap, true); fieldTokenSwap = new ArrayList<>(); currentTokenBuilder = new StringBuilder(); fieldTokenSwap.add(currentTokenBuilder); break; } currentTokenBuilder.append(currentChar); break; case ' ': case '\t': case '\r': case '\n': if(deep > 0) { currentTokenBuilder.append(currentChar); continue; } if(currentTokenBuilder.length() == 0) { continue; } // original_name as --> alias if(i + 1 < len) { int j = i + 1; // 收集连续的空格 StringBuilder spaceHolder = new StringBuilder(); boolean isNextLeftBracket = false; do { char nextChar = columnPart.charAt(j++); if(nextChar == ' ' || nextChar == '\t' || nextChar == '\r' || nextChar == '\n') { spaceHolder.append(nextChar); continue; } if(nextChar == '(') { isNextLeftBracket = true; } break; } while (j < len); if(isNextLeftBracket) { currentTokenBuilder.append(currentChar); } if(spaceHolder.length() > 0) { currentTokenBuilder.append(spaceHolder); i += spaceHolder.length(); } if(isNextLeftBracket) { // continue next for, function begin continue; } } if(fieldTokenSwap.size() == 1) { if(fieldTokenSwap.get(0).toString().equalsIgnoreCase("case")) { String caseWhenPart = CommonUtil.readSplitWord( columnPartChars, i, " ", "end"); currentTokenBuilder.append(caseWhenPart); if(caseWhenPart.length() <= 0) { throw new BizException("语法错误，未找到case..when的结束符"); } i += caseWhenPart.length(); } } addNewField(fieldList, fieldTokenSwap, false); currentTokenBuilder = new StringBuilder(); fieldTokenSwap.add(currentTokenBuilder); break; // 空格忽略 default: currentTokenBuilder.append(currentChar); break; } } // 处理剩余尚未存储的字段信息 addNewField(fieldList, fieldTokenSwap, true); return fieldList; } /** * 新增一个字段描述 * * @param fieldList 字段容器 * @param fieldTokenSwap 候选词 */ private static void addNewField(List<SelectFieldClauseDescriptor> fieldList, List<StringBuilder> fieldTokenSwap, boolean forceAdd) { int ts = fieldTokenSwap.size(); if(ts == 1 && forceAdd) { // db.original_name, String fieldName = fieldTokenSwap.get(0).toString(); String alias = fieldName; if(fieldName.contains(".")) { alias = fieldName.substring(fieldName.lastIndexOf('.') + 1); } fieldList.add(new SelectFieldClauseDescriptor(fieldName, alias)); return; } if(ts < 2) { return; } if(ts == 2) { // original_name alias, if(fieldTokenSwap.get(1).toString().equalsIgnoreCase("as")) { return; } fieldList.add(new SelectFieldClauseDescriptor( fieldTokenSwap.get(0).toString(), fieldTokenSwap.get(1).toString())); } else if(ts == 3) { // original_name as alias, fieldList.add(new SelectFieldClauseDescriptor( fieldTokenSwap.get(0).toString(), fieldTokenSwap.get(2).toString())); } else { throw new BizException("字段语法解析错误，超过3个以字段描述信息:" + ts); } } // 截取适配 field 字段信息部分 private static String adaptFieldPartSql(String fullSql) { int start = fullSql.lastIndexOf("select "); int end = fullSql.lastIndexOf(" from"); String columnPart = fullSql.substring(start + "select ".length(), end); return columnPart.trim(); } }
应该说是比较简单的，一个for, 一个 switch ，就搞定了。其他的，更多的是逻辑判定。
下面我们来看看字段描述类的写法，其实就是两个字段，源字段和别名。
/** * 功能描述: sql字段描述 select 字段描述类 * */ public class SelectFieldClauseDescriptor { private String fieldName; private String alias; public SelectFieldClauseDescriptor(String fieldName, String alias) { this.fieldName = fieldName; this.alias = alias; } public String getFieldName() { return fieldName; } public String getAlias() { return alias; } @Override public boolean equals(Object o) { if (this == o) return true; if (o == null || getClass() != o.getClass()) return false; SelectFieldClauseDescriptor that = (SelectFieldClauseDescriptor) o; return Objects.equals(fieldName, that.fieldName) && Objects.equals(alias, that.alias); } @Override public int hashCode() { return Objects.hash(fieldName, alias); } @Override public String toString() { return "SelectFieldClauseDescriptor{" + "fieldName='" + fieldName + '\'' + ", alias='" + alias + '\'' + '}'; } }
它存在的意义，仅仅是为了使用方更方便取值，以为更进一步的解析提供了依据。
3. 单元测试
其实像写这种工具类，最是方便简单。因为最初的结果，我们早已预料，以测试驱动开发最合适不过了。而且，基本上一出现不符合预期的值时，很快速就定位问题了。
/** * 功能描述: sql字段解析器测试 **/ public class SimpleSqlFieldParserTest { @Test public void testParse() { String sql; List<SelectFieldClauseDescriptor> parsedFieldList; sql = "select COALESCE(t1.xno, t2.xno, t3.xno) as xno,\n" + " case when t1.xno is not null then 1 else null end as xxk001,\n" + " case when t2.xno is not null then 1 else null end as xxk200,\n" + " case when t3.xno is not null then 1 else null end as xx3200\n" + " from xxk001 t1\n" + " full join xxkj100 t2 on t1.xno = t2.xno\n" + " full join xxkj200 t3 on t1.xno = t3.xno;"; parsedFieldList = SimpleSqlFieldParser.parse(sql); System.out.println("result:"); parsedFieldList.forEach(System.out::println); Assert.assertEquals("字段个数解析不正确", 4, parsedFieldList.size()); Assert.assertEquals("字段别名解析不正确", "xno", parsedFieldList.get(0).getAlias()); Assert.assertEquals("字段别名解析不正确", "xx3200", parsedFieldList.get(3).getAlias()); sql = "select cast(a as string) as b from ccc;"; parsedFieldList = SimpleSqlFieldParser.parse(sql); System.out.println("result:"); parsedFieldList.forEach(System.out::println); Assert.assertEquals("字段个数解析不正确", 1, parsedFieldList.size()); Assert.assertEquals("字段别名解析不正确", "b", parsedFieldList.get(0).getAlias()); sql = "with a as(select cus,x1 from b1), b as (select cus,x2 from b2)\n" + " select a.cus as a_cus, cast(a \nas string) as a_cus2, " + "b.x2 b2 from a join b on a.cus=b.cus where xxx;"; parsedFieldList = SimpleSqlFieldParser.parse(sql); System.out.println("result:"); parsedFieldList.forEach(System.out::println); Assert.assertEquals("字段个数解析不正确", 3, parsedFieldList.size()); Assert.assertEquals("字段别名解析不正确", "a_cus", parsedFieldList.get(0).getAlias()); Assert.assertEquals("字段别名解析不正确", "b2", parsedFieldList.get(2).getAlias()); sql = "select a.xno,b.xx,qqq from a_tb as a join b_tb as b on a.id = b.id"; parsedFieldList = SimpleSqlFieldParser.parse(sql); System.out.println("result:"); parsedFieldList.forEach(System.out::println); Assert.assertEquals("字段个数解析不正确", 3, parsedFieldList.size()); Assert.assertEquals("字段别名解析不正确", "xno", parsedFieldList.get(0).getAlias()); Assert.assertEquals("字段别名解析不正确", "qqq", parsedFieldList.get(2).getAlias()); sql = "select cast (a.a_int as string) a_str, b.xx, coalesce \n( a, b, c) qqq from a_tb as a join b_tb as b on a.id = b.id"; parsedFieldList = SimpleSqlFieldParser.parse(sql); System.out.println("result:"); parsedFieldList.forEach(System.out::println); Assert.assertEquals("字段个数解析不正确", 3, parsedFieldList.size()); Assert.assertEquals("字段别名解析不正确", "a_str", parsedFieldList.get(0).getAlias()); Assert.assertEquals("字段原始名解析不正确", "cast (a.a_int as string)", parsedFieldList.get(0).getFieldName()); Assert.assertEquals("字段别名解析不正确", "qqq", parsedFieldList.get(2).getAlias()); Assert.assertEquals("字段原始名解析不正确", "coalesce \n( a, b, c)", parsedFieldList.get(2).getFieldName()); } }
至此，一个简单的字段解析器完成。小工具，供参考！