OceanBase 数据库大赛：支持NULL类型(草稿)

程序员小王

发布于 2021-11-18 14:12:49

1.7K00

代码可运行

文章被收录于专栏：算法之美算法之美

运行总次数：0

代码可运行

题目

https://github.com/OceanBase-Partner/lectures-on-dbms-implementation/blob/main/miniob-topics.md

调试


少记录

breakpoint set --file condition_filter.cpp --line 388

breakpoint set --file condition_filter.cpp --line 270

breakpoint set --file bplus_tree.cpp --line 2467



查询索引
IndexScanner *Table::find_index_for_scan

IndexScanner *BplusTreeIndex::create_scanner
BplusTreeScanner::open
find_first_index_satisfied
breakpoint set --file bplus_tree.cpp --line 190
}

Table::scan_record

scan_record_by_index

next_entry

get_next_idx_in_memory
filter->filter
bool BplusTreeScanner::satisfy_condition

breakpoint set --file bplus_tree.cpp --line 2283


breakpoint set --file tuple.cpp --line 531

breakpoint set --file tuple.cpp --line 602


breakpoint set --file bplus_tree.cpp --line 2274

breakpoint set --file bplus_tree.cpp --line 2309


breakpoint set --file bplus_tree.cpp --line 190

breakpoint set --file tuple.cpp --line 531


breakpoint set --file condition_filter.cpp --line 105


breakpoint set --file execute_stage.cpp --line 811


breakpoint set --file execute_stage.cpp --line 722


case SCF_INSERT:

breakpoint set --file default_storage_stage.cpp --line 180


breakpoint set --file execute_stage.cpp  --line 340


breakpoint set --file tuple.cpp --line 586


breakpoint set --file condition_filter.cpp --line 143


breakpoint set --file condition_filter.cpp --line 105


breakpoint set --file execute_stage.cpp --line 811


breakpoint set --file execute_stage.cpp --line 722




IndexScanner *Table::find_index_for_scan(const DefaultConditionFilter &filter)


breakpoint set --file bplus_tree.cpp --line 190


breakpoint set --file table.cpp --line 1006


bool BplusTreeScanner::satisfy_condition(const char *pkey)

breakpoint set --file bplus_tree.cpp --line 2270

breakpoint set --file bplus_tree.cpp --line 1991

难点

1.5

问：多表查询，case：select t12.num,t13.num,t12.birthday from t12,t13 where t12.num=t13.num; 显示时候 select t12.num,t13.num,t12.birthday 你们思路怎么处理的，默认代码按照表过滤字段显示的？

f7327f5bb81fada1d222df3b9e8a82e6.png

1.4gdb 对比普通索引和null 索引查找差异

提示1：如果更快全集搜索之前运算符是怎么解决的 case LESS_EQUAL: ｜｜ case NOT_EQUAL: ｜｜
提示2:比较运算符的差异 select * from t12 where num =14; select * from t12 where num is null;

查询索引
IndexScanner *Table::find_index_for_scan

IndexScanner *BplusTreeIndex::create_scanner
BplusTreeScanner::open
find_first_index_satisfied
breakpoint set --file bplus_tree.cpp --line 190
}

Table::scan_record

scan_record_by_index

next_entry

get_next_idx_in_memory
filter->filter
bool BplusTreeScanner::satisfy_condition

breakpoint set --file bplus_tree.cpp --line 2283

我gdb跟踪错误：数据为什么不对。结果发现：

breakpoint set --file condition_filter.cpp --line 388

breakpoint set --file condition_filter.cpp --line 270

breakpoint set --file bplus_tree.cpp --line 2467

bool DefaultConditionFilter::filter(const Record &rec) const

1.3 有索引比较和无索引比较

bool DefaultConditionFilter::filter(const Record &rec) const
CompareKey
bool BplusTreeScanner::satisfy_condition(const char *pkey)

void TupleRecordConverter::add_record(const char *record)

潜台词：这三个函数干什么的，

1.2 null 与值，运算符，null比较

null 与null true
null 与其他值 false

1.3 null 怎么存储自己用999代替

0 == strcmp(s, "999")
null 在内存地址怎么存储，用null ptr 吗？解析不到就是 null值？

思路分析

第19次测试

无索引，无过滤条件，查询

bool DefaultConditionFilter::filter(const Record &rec) const

execute_stage.cpp


breakpoint set --file execute_stage.cpp --line 342


breakpoint set --file condition_filter.cpp --line 388

breakpoint set --file condition_filter.cpp --line 388

breakpoint set --file condition_filter.cpp --line 270

breakpoint set --file bplus_tree.cpp --line 2467

8c2f3f2f0768518da9706a4dcf8cac6d.png

select NUM from NULL_TABLE3;

breakpoint set --file bplus_tree.cpp --line 2467

问题关键地方：

查询无记录 select * from NULL_TABLE3;

自己开始认为null ，不需要查询出来【错误】

case :在纯null情况下 avg函数出现了问题

select * from NULL_TABLE3;

breakpoint set --file execute_stage.cpp --line 342

breakpoint set --file tuple.cpp --line 355

breakpoint set --file tuple.cpp --line 834

839

SELECT NUM FROM NULL_TABLE3;

null: result file difference(`-` is yours and `+` is base)
 CREATE TABLE NULL_TABLE3(ID INT, NUM INT NULLABLE);
 SUCCESS
 INSERT INTO NULL_TABLE3 VALUES (1, NULL);
 SUCCESS
 INSERT INTO NULL_TABLE3 VALUES (2, NULL);
 SUCCESS
 SELECT COUNT(NUM) FROM NULL_TABLE3;
 COUNT(NUM)
 0
 SELECT MIN(NUM) FROM NULL_TABLE3;
+MIN(NUM)
+NULL
+SELECT max(num) FROM null_table3;
+MAX(NUM)
+NULL
+SELECT avg(num) FROM null_table3;
+AVG(NUM)
+NULL

8f306a2f07ef2203d29b6045fdb169ad.png

第18次测试

2359ac666e468f2b549276eeb91efa28.png

第17次测试：多表情况

insert into t13 values(1,20,"2021-01-30");
insert into t12 values(2,20,"2021-01-01");
select t12.num,t13.num,t12.birthday from t12,t13 where t12.num=t13.num;

breakpoint set --file execute_stage.cpp --line 308

selects

[0] = (relation_name = "t12", attribute_name = "birthday", funtype = FUN_NO) [1] = (relation_name = "t13", attribute_name = "num", funtype = FUN_NO) [2] = (relation_name = "t12", attribute_name = "num", funtype = FUN_NO)

问：多表查询，case：select t12.num,t13.num,t12.birthday from t12,t13 where t12.num=t13.num;

显示时候 select t12.num,t13.num,t12.birthday 你们思路怎么处理的，默认代码按照表过滤字段显示的？

select t12.num,t13.num,t12.birthday from t12,t13 where t12.num=t13.num;

4f12ecdddfc3c75ec196989c93fe0313.png

breakpoint set --file execute_stage.cpp --line 307

breakpoint set --file execute_stage.cpp --line 270

breakpoint set --file execute_stage.cpp --line 340

breakpoint set --file execute_stage.cpp --line 362

1fdce6189431146436486957ee986a74.png

 schema_ = {
      fields_ = size=2 {
        [0] = {
          type_ = INTS
          table_name_ = "t12"
          field_name_ = "num"
          visible_ = true
          function_type = FUN_NO
          nullable_ = 1
          field_name_count_number_ = ""
        }
        [1] = {
          type_ = DATES
          table_name_ = "t12"
          field_name_ = "birthday"
          visible_ = true
          function_type = FUN_NO
          nullable_ = 1
          field_name_count_number_ = ""
        }
      }
      realTabeNumber = -1
    }
    
   [0] = {
    tuples_ = size=1 {
      [0] = {
        values_ = size=1 {
          [0] = std::__1::shared_ptr<TupleValue>::element_type @ 0x00007fc448604080 strong=1 weak=1 {
            __ptr_ = 0x00007fc448604080
          }
        }
        sp1 = nullptr {
          __ptr_ = nullptr
        }
        sp2 = nullptr {
          __ptr_ = nullptr
        }
        sp3 = nullptr {
          __ptr_ = nullptr
        }
        selectComareIndex = -1
      }
    }
    schema_ = {
      fields_ = size=1 {
        [0] = {
          type_ = INTS
          table_name_ = "t13"
          field_name_ = "num"
          visible_ = true
          function_type = FUN_NO
          nullable_ = 1
          field_name_count_number_ = ""
        }
      }
      realTabeNumber = -1
    }

判断是否调整
调整


select t12.num,t13.num,t12.birthday from t12,t13 where t12.num=t13.num;

create_selection_executor
null: result file difference(`-` is yours and `+` is base)
 SELECT NULL_TABLE.NUM,NULL_TABLE2.NUM,NULL_TABLE.BIRTHDAY FROM NULL_TABLE,NULL_TABLE2 WHERE NULL_TABLE.NUM=NULL_TABLE2.NUM;
-18 | 2020-01-01 | 18
-NULL_TABLE.NUM | NULL_TABLE.BIRTHDAY | NULL_TABLE2.NUM
+18 | 18 | 2020-01-01
+NULL_TABLE.NUM | NULL_TABLE2.NUM | NULL_TABLE.BIRTHDAY

16次测试：

有索引情况下：非null，null与值比较为false，一个记录无都查询不到呀？这个怎么处理 select * from t12 where num is not null; ---应该有记录的

insert into t12 values(6,null,null); //0k

select * from t12 where num =14; select * from t12 where num is null; select * from t12 where num is not null;

15 有错了

修改：bool DefaultConditionFilter::filter

普通类型和int 如何比较

gdb 调试


breakpoint set --file execute_stage.cpp --line 270

breakpoint set --file execute_stage.cpp --line 397

select * from t12 where num is not null;

1 | 2 | 2020-01-01 缺少记录呢

看日志：DefaultConditionFilter::filter IS_NOT_NULL

void TupleRecordConverter::add_record(const char *record)

索引查询：

1999行

tmp = CompareKey(node->keys + i * file_header_.key_length, key, file_header_.attr_type, file_header_.attr_length);

case LESS_THAN:
    switch (attr_type)
    {
    case INTS:
      flag = (i1 < i2);
      break;
    case FLOATS:
      flag = (f1 < f2);
      break;
    case CHARS:
      flag = (strncmp(s1, s2, attr_length) < 0);
      break;
    case DATES:
      flag = (i1 < i2);
      break;
    default:
      LOG_PANIC("Unknown attr type: %d", attr_type);
    }
    break;

有错了，在有索引和无索引条件比较不对

根据测试我写代码不对

int CompareKey(const char *pdata, const char *pkey, AttrType attr_type, int attr_length)

4 对比代码

* thread #9, stop reason = breakpoint 1.1
  * frame #0: 0x0000000109e572e4 observer`CompareKey(pdata="\U00000002", pkey="999", attr_type=INTS, attr_length=4) at bplus_tree.cpp:191:3 [opt]
    frame #1: 0x0000000109e5d05a observer`BplusTreeHandler::find_first_index_satisfied(this=0x00007ff5b1407788, compop=IS_NULL, key="\xff\xff\xff\xff", page_num=<unavailable>, rididx=0x00007ff5b2b04804) at bplus_tree.cpp:1955:13 [opt]
    frame #2: 0x0000000109e5d3cf observer`BplusTreeScanner::open(this=0x00007ff5b2b044c0, comp_op=IS_NULL, value=<unavailable>) at bplus_tree.cpp:2081:23 [opt]
    frame #3: 0x0000000109e60445 observer`BplusTreeIndex::create_scanner(this=0x00007ff5b14076e0, comp_op=IS_NULL, value="999") at bplus_tree_index.cpp:98:31 [opt]
    frame #4: 0x0000000109e76550 observer`Table::find_index_for_scan(this=0x00007ff5b1605160, filter=<unavailable>) at table.cpp:1067:31 [opt]
    frame #5: 0x0000000109e75f54 observer`Table::scan_record(this=0x00007ff5b1605160, trx=0x00007ff5b1606eb0, filter=0x00007000065b1818, limit=2147483647, context=0x00007000065b17e0, record_reader=(observer`scan_record_reader_adapter(Record*, void*) at table.cpp:502))(Record*, void*)) at table.cpp:531:33 [opt]

结果：

[2021-11-13 18:44:3 pid:18738 tid:7000065b5000  INFO: /Users/wangchuanyi/code/miniob/src/observer/sql/parser/parse.cpp condition_init 195]>>left_is_attr ...........
[2021-11-13 18:44:3 pid:18738 tid:7000065b5000  INFO: /Users/wangchuanyi/code/miniob/src/observer/storage/common/bplus_tree.cpp CompareKey 197]>>CompareKey int >>>: 1=2,2=3750201
[2021-11-13 18:44:3 pid:18738 tid:7000065b5000  INFO: /Users/wangchuanyi/code/miniob/src/observer/storage/common/bplus_tree.cpp CompareKey 197]>>CompareKey int >>>: 1=2,2=3750201
[2021-11-13 18:44:3 pid:18738 tid:7000065b5000  INFO: /Users/wangchuanyi/code/miniob/src/observer/storage/common/bplus_tree.cpp CompareKey 197]>>CompareKey int >>>: 1=2,2=3750201
[2021-11-13 18:44:3 pid:18738 tid:7000065b5000  INFO: /Users/wangchuanyi/code/miniob/src/observer/storage/common/bplus_tree.cpp CompareKey 197]>>CompareKey int >>>: 1=3750201,2=3750201
[2021-11-13 18:44:3 pid:18738 tid:7000065b5000  INFO: /Users/wangchuanyi/code/miniob/src/observer/storage/common/bplus_tree.cpp CompareKey 197]>>CompareKey int >>>: 1=3750201,2=3750201
[2021-11-13 18:44:3 pid:18738 tid:7000065b5000  INFO: /Users/wangchuanyi/code/miniob/src/observer/storage/common/bplus_tree.cpp CompareKey 197]>>CompareKey int >>>: 1=3750201,2=3750201
[2021-11-13 18:44:3 pid:18738 tid:7000065b5000  INFO: /Users/wangchuanyi/code/miniob/src/observer/storage/common/bplus_tree.cpp CompareKey 197]>>CompareKey int >>>: 1=3750201,2=3750201
[2021-11-13 18:44:3 pid:18738 tid:7000065b5000  INFO: /Users/wangchuanyi/code/miniob/src/observer/sql/executor/execute_stage.cpp do_select 681]>> query resault. id | num | birthday

原因呢？
插入的时候null 是按照999存储的。但是查找过程中。自己没有修改解析，null是按照 null过滤的

//比较2个key 大小
int CompareKey(const char *pdata, const char *pkey, AttrType attr_type, int attr_length)

bool DefaultConditionFilter::filter(const Record &rec) const

不对呀比较从哪里跌

根据索引查找：

RC Table::scan_record(

null 用 999 表示

 IndexScanner *index_scanner = find_index_for_scan(filter);
  if (index_scanner != nullptr)
  {
    return scan_record_by_index(trx, index_scanner, filter, limit, context, record_reader);
  }

1faecc37091dddb49caa28920dbbaaf6.png

2.11 - null 与null 比较规规则和上买的相反

修改过滤条件：

bool DefaultConditionFilter::filter(const Record &rec) const

2.10 有错误了

题目：支持NULL类型

sql：select * from t where null is null; -- 注意 = 与 is 的区别

问题：null is null 这个是true 还是false呀？

测试结果不对

d4eb03443d80d58b67138d2bab9c486f.png

错误代码位置：null 不是列直接报错
为什么 NULLL 变了？？？

e9f6fd4ac322baa76fb9d1189723bc41.png

right_is_attr = 1 right_attr = (relation_name = 0x0000000000000000, attribute_name = "NULLL", funtype = FUN_MAX | 0xe2f300a8)

24d1a4919d6dbd8c241f423396d1bf3d.png

错误sql：

select * from t12 WHERE NULL IS NULLL;

right.is_attr = true;
    const FieldMeta *field_right = table_meta.field(condition.right_attr.attribute_name);
    if (nullptr == field_right)
    {
      LOG_WARN("No such field in condition. %s.%s", table.name(), condition.right_attr.attribute_name);
      return RC::SCHEMA_FIELD_MISSING;
    }
    
    
  void condition_init(Condition *condition, CompOp comp, 
                    int left_is_attr, RelAttr *left_attr, Value *left_value,
                    int right_is_attr, RelAttr *right_attr, Value *right_value) {

|value comOp value

2.9 比较运算符与null比较都是false

Comparison Functions and Operators

https://dev.mysql.com/doc/refman/8.0/en/working-with-null.html

12.4.2 Comparison Functions and Operators

https://dev.mysql.com/doc/refman/8.0/en/comparison-operators.html#operator_equal

https://dev.mysql.com/doc/refman/8.0/en/working-with-null.html

mysql> SELECT 1 = NULL, 1 <> NULL, 1 < NULL, 1 > NULL;

You cannot use arithmetic comparison operators such as =, <, or <> to test for NULL. To demonstrate this for yourself, try the following query:
Because the result of any arithmetic comparison with NULL is also NULL, you cannot obtain any meaningful results from such comparisons. https://dev.mysql.com/doc/refman/8.0/en/working-with-null.html

画外音：

任何值与NULL做对比，结果都是FALSE。
任何比较运算 NULL做对比，结果都是FALSE。

代码：

bool DefaultConditionFilter::filter(const Record &rec) const

select * from t12 where 1<>NULL; select * from t12 where NULL<>1;

filter->filter(record)))

RC Table::scan_record_by_index(

class ConditionFilter {
public:
  virtual ~ConditionFilter();

  /**
   * Filter one record
   * @param rec
   * @return true means match condition, false means failed to match.
   */
  virtual bool filter(const Record &rec) const = 0;
};

[ ] 完整查看select 过滤条件

do_select->create_selection_executor->

class DefaultConditionFilter : public ConditionFilter

execute

//sql执行
RC SelectExeNode::execute(TupleSet &tuple_set) {
  CompositeConditionFilter condition_filter;
  condition_filter.init((const ConditionFilter **)condition_filters_.data(), condition_filters_.size());

  tuple_set.clear();
  tuple_set.set_schema(tuple_schema_);
  tuple_set.old_schema =old_tuple_schema;
  TupleRecordConverter converter(table_, tuple_set);
  return table_->scan_record(trx_, &condition_filter, -1, (void *)&converter, record_reader);
}

RC Table::scan_record(Trx *trx, ConditionFilter *filter, int limit, void *context, RC (*record_reader)(Record *record, void *context))
IndexScanner *index_scanner = find_index_for_scan(filter);
rc = scanner.open_scan(*data_buffer_pool_, file_id_, filter);
condition_filter_->filter(current_record))
bool DefaultConditionFilter::filter(const Record &rec) const

总结：

任何值（保留运算符）与NULL做对比，结果都是FALSE

https://dev.mysql.com/doc/refman/8.0/en/working-with-null.html

2.8 修改索引过滤条件

bool BplusTreeScanner::satisfy_condition(const char *pkey)

AttrType attr_type = index_handler_.file_header_.attr_type;

 case IS_NULL:
    {
      flag =false;
    }
    break;

    case IS_NOT_NULL:
    {
      flag =true;
    }
    break;

2.7 为什么执行无结果

select * from t12 where id is null; select * from t12 where id is not null;

select * from t12 where 1<>NULL; select * from t12 where 1 IS NOT NULL;

任务：RC DefaultConditionFilter::init(Table &table, const Condition &condition)

[ ] 任务：

bool DefaultConditionFilter::filter(const Record &rec) const

1 IS NOT NULL; //true case IS_NOT_NULL: return cmp_result != 0;

2.6 执行错误

select * from t12 where id is null;

std::vector<DefaultConditionFilter *> condition_filters;

RC DefaultConditionFilter::init(Table &table, const Condition &condition)

private:
  ConDesc  left_;
  ConDesc  right_;
  AttrType attr_type_ = UNDEFINED;
  CompOp   comp_op_ = NO_OP;
};

任务：调整RC DefaultConditionFilter::init函数

2.5 增加查询时候 null 判断

where:
condition:

2.4 create: 增加 NOT NULL 关键字

2.3 修改聚合函数

bool TupleSet::avg_print(std::ostream &os) const

TupleField 包含聚合函数
std::vector<std::shared_ptr> values_;

2.2 显示 Tuple tuple;

null 显示和普通类型的显示一样吗？
这里考虑单行的情况
//单表聚合:只有一行
看懂类：class Tuple

代码实现

2.3 where条件

产生过滤条件：std::vector<DefaultConditionFilter *> condition_filters;

RC DefaultConditionFilter::init(Table &table, const Condition &condition)

private:
  ConDesc  left_;
  ConDesc  right_;
  AttrType attr_type_ = UNDEFINED;
  CompOp   comp_op_ = NO_OP;
};

执行过滤条件
bool DefaultConditionFilter::filter(const Record &rec) const

bool BplusTreeScanner::satisfy_condition(const char *pkey)

用 999代码

comOp:
     EQ { CONTEXT->comp = EQUAL_TO; }
    | LT { CONTEXT->comp = LESS_THAN; }
    | GT { CONTEXT->comp = GREAT_THAN; }
    | LE { CONTEXT->comp = LESS_EQUAL; }
    | GE { CONTEXT->comp = GREAT_EQUAL; }
    | NE { CONTEXT->comp = NOT_EQUAL; }
    ;

2.2 显示 Tuple tuple;

void TupleRecordConverter::add_record(const char *record)

测试结果

case 3

null: result file difference(`-` is yours and `+` is base)
 3. SELECT WITH CONSTANT
 SELECT * FROM NULL_TABLE WHERE 1 IS NULL;
-FAILURE
+ID | NUM | PRICE | BIRTHDAY
 SELECT * FROM NULL_TABLE WHERE 1 IS NOT NULL;
-FAILURE
+1 | 18 | 10 | 2020-01-01
+2 | NULL | 20 | 2010-01-11
+3 | 12 | 30 | NULL
+4 | 15 | 30 | 2021-01-31
+ID | NUM | PRICE | BIRTHDAY
 SELECT * FROM NULL_TABLE WHERE NULL=1;
-FAILURE
+ID | NUM | PRICE | BIRTHDAY
 SELECT * FROM NULL_TABLE WHERE 1=NULL;
-FAILURE
+ID | NUM | PRICE | BIRTHDAY
 SELECT * FROM NULL_TABLE WHERE 1<>NULL;
-FAILURE
+ID | NUM | PRICE | BIRTHDAY
 SELECT * FROM NULL_TABLE WHERE 1<NULL;
-FAILURE
+ID | NUM | PRICE | BIRTHDAY
 SELECT * FROM NULL_TABLE WHERE 1>NULL;
-FAILURE
+ID | NUM | PRICE | BIRTHDAY

CREATE TABLE NULL_TABLE(ID INT, NUM INT NULLABLE, PRICE FLOAT NOT NULL, BIRTHDAY DATE NULLABLE);

CREATE TABLE NULL_TABLE2(ID INT, NUM INT NULLABLE, PRICE FLOAT NOT NULL, BIRTHDAY DATE NULLABLE);

CREATE INDEX INDEX_NUM ON NULL_TABLE(NUM);


 CREATE TABLE NULL_TABLE(ID INT, NUM INT NULLABLE, PRICE FLOAT NOT NULL, BIRTHDAY DATE NULLABLE);
-FAILURE
+SUCCESS
 CREATE TABLE NULL_TABLE2(ID INT, NUM INT NULLABLE, PRICE FLOAT NOT NULL, BIRTHDAY DATE NULLABLE);
-FAILURE
+SUCCESS
 CREATE INDEX INDEX_NUM ON NULL_TABLE(NUM);
-FAILURE
+SUCCESS

3298bb857adc370ead1071015fc2060b.png

core ok

: line 1: 1355 Segmentation fault: 11 ./observer -f ./observer.ini

0x0000000107d8ef8d Table::make_record(int, _Value const*, char*&) + 1357 (table.cpp:430)

不对 ok

f93af99ea8021b6cb8df592825469889.png

null: result file difference(`-` is yours and `+` is base)
 3. SELECT WITH CONSTANT
 SELECT * FROM NULL_TABLE WHERE 1 IS NULL;
 ID | NUM | PRICE | BIRTHDAY
 SELECT * FROM NULL_TABLE WHERE 1 IS NOT NULL;
 1 | 18 | 10 | 2020-01-01
 2 | NULL | 20 | 2010-01-11
 3 | 12 | 30 | NULL
 4 | 15 | 30 | 2021-01-31
 ID | NUM | PRICE | BIRTHDAY
 SELECT * FROM NULL_TABLE WHERE NULL=1;
-FAILURE
+ID | NUM | PRICE | BIRTHDAY
 SELECT * FROM NULL_TABLE WHERE 1=NULL;
 ID | NUM | PRICE | BIRTHDAY
 SELECT * FROM NULL_TABLE WHERE 1<>NULL;
-1 | 18 | 10 | 2020-01-01
-2 | NULL | 20 | 2010-01-11
-3 | 12 | 30 | NULL
-4 | 15 | 30 | 2021-01-31
 ID | NUM | PRICE | BIRTHDAY
 SELECT * FROM NULL_TABLE WHERE 1<NULL;
-1 | 18 | 10 | 2020-01-01
-2 | NULL | 20 | 2010-01-11
-3 | 12 | 30 | NULL
-4 | 15 | 30 | 2021-01-31
 ID | NUM | PRICE | BIRTHDAY
 SELECT * FROM NULL_TABLE WHERE 1>NULL;
 ID | NUM | PRICE | BIRTHDAY

本文参与腾讯云自媒体同步曝光计划，分享自微信公众号。

原始发表：2021-11-16，如有侵权请联系 cloudcommunity@tencent.com 删除

c++

编程算法

linux

https

本文分享自 Offer多多微信公众号，前往查看

如有侵权，请联系 cloudcommunity@tencent.com 删除。

本文参与腾讯云自媒体同步曝光计划，欢迎热爱写作的你一起参与！

登录后参与评论

0 条评论

热度