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  • MySql获取一个最小的空ID

    id列中的已有的数据1,2,7,8,9, 业务需要从id列中找出一个最小的没占用的, 上面应该查出的结果是3.

    例:

    select @rowNum:=@rowNum + 1 AS rn,a.id FROM tabName a,(select @rowNum:=0) b order by id asc
    

     查询列图

    上面查询行号与id, 只要查询rn=6的值就行了, id>rn

    select rn from (select @rowNum:=@rowNum + 1 AS rn,a.id FROM tabName a,(select @rowNum:=0) b order by id asc) c where id > rn limit 1
    

      上面没考虑到数据库中id与正好是1,2,3,4, 有序的, 一个不缺的, 修改如下:

        /**
         * 获取一个空的id
         * @param jdbcTemplate
         * @param tabName
         * @param strIDFieldName
         * @return
         */
        public static int getEmptyID(JdbcTemplate jdbcTemplate, String tabName, String strIDFieldName) {
            String sql = "select rn from (select @rowNum:=@rowNum + 1 AS rn,a.id FROM "
                    + tabName + " a,(select @rowNum:=0) b order by " + strIDFieldName + " asc) c where " + strIDFieldName + " > rn limit 1";
            Integer id = jdbcTemplate.queryIntOrNull(sql);
            if(id == null) {
                id = jdbcTemplate.queryInt("select max(`" + strIDFieldName + "`) from " + tabName);
                ++id;
            }
            if(id > 0) {
                return id;
            } else {
                return 1;
            }
        }
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  • 原文地址:https://www.cnblogs.com/barrysgy/p/14550450.html
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