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，单元测试最是方便简单。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());    }}