以mysql为例，update语法如下：官网地址

UPDATE [LOW_PRIORITY] [IGNORE] table_reference
    SET assignment_list
    [WHERE where_condition]
    [ORDER BY ...]
    [LIMIT row_count]

value:
    {expr | DEFAULT}

assignment:
    col_name = value

assignment_list:
    assignment [, assignment] ...

UPDATE [LOW_PRIORITY] [IGNORE] table_references
    SET assignment_list
    [WHERE where_condition]

update语句经过语法解析器，最终会交由AbstractUpdateParser的parser方法去处理。前面过程请参考上篇博客。

public abstract class AbstractUpdateParser implements SQLParser {
    
    private final ShardingRule shardingRule;
    
    private final LexerEngine lexerEngine;
    
    private final AbstractUpdateClauseParserFacade updateClauseParserFacade;
    
    @Override
    public DMLStatement parse() {
        lexerEngine.nextToken();
        lexerEngine.skipAll(getSkippedKeywordsBetweenUpdateAndTable());
        lexerEngine.unsupportedIfEqual(getUnsupportedKeywordsBetweenUpdateAndTable());
        DMLStatement result = new DMLStatement();
          //解析表引用关系 
        updateClauseParserFacade.getTableReferencesClauseParser().parse(result, true);
        //解释update语句
        updateClauseParserFacade.getUpdateSetItemsClauseParser().parse(result);
        lexerEngine.skipUntil(DefaultKeyword.WHERE);
        //update语句的where条件处理           
        updateClauseParserFacade.getWhereClauseParser().parse(shardingRule, result, Collections.<SelectItem>emptyList());
        return result;
    }
    
    protected Keyword[] getSkippedKeywordsBetweenUpdateAndTable() {
        return new Keyword[0];
    }
    
    protected Keyword[] getUnsupportedKeywordsBetweenUpdateAndTable() {
        return new Keyword[0];
    }
}

where条件解析和表引用关系解析在select语句处理的时候解释过，这里不再细看。

public void parse(final DMLStatement updateStatement) {
    //接受set关键词
    lexerEngine.accept(DefaultKeyword.SET);
    do {
        parseSetItem(updateStatement);
        //如果是，则循环处理
    } while (lexerEngine.skipIfEqual(Symbol.COMMA));
}

private void parseSetItem(final DMLStatement updateStatement) {
    //列名解析
    parseSetColumn(updateStatement);
    //判断是否是=，或者是 :=
    lexerEngine.skipIfEqual(Symbol.EQ, Symbol.COLON_EQ);
    //列名对应value值解析
    parseSetValue(updateStatement);
    //跳过::
    skipsDoubleColon();
}

private void parseSetColumn(final DMLStatement updateStatement) {
    if (lexerEngine.equalAny(Symbol.LEFT_PAREN)) {
        lexerEngine.skipParentheses(updateStatement);
        return;
    }
    int beginPosition = lexerEngine.getCurrentToken().getEndPosition();
    String literals = lexerEngine.getCurrentToken().getLiterals();
    lexerEngine.nextToken();
    //如果有表的别名
    if (lexerEngine.skipIfEqual(Symbol.DOT)) {
        if (updateStatement.getTables().getSingleTableName().equalsIgnoreCase(SQLUtil.getExactlyValue(literals))) {
            updateStatement.getSqlTokens().add(new TableToken(beginPosition - literals.length(), 0, literals));
        }
        lexerEngine.nextToken();
    }
}
  
private void parseSetValue(final DMLStatement updateStatement) {
    basicExpressionParser.parse(updateStatement);
}