MySQL查詢語句系列(一)

1 數據表數據

圖1 數據表數據

2 sum

• 功能
  數據求和統計.

序號	表達式	描述
1	sum(column_name)	所有列的列值相加
2	sum(expression)	統計滿足表達式的所有行

2.1 sum(column_name)

• 查詢語句:

select sum(id) from userinfos group by sex;

• 查詢結果:

+---------+
| sum(id) |
+---------+
|     166 |
|      36 |
+---------+

2.2 sum(experssion)

• 查詢語句:

select sum(position="CEO") from userinfos group by sex;

• 查詢結果:

+---------------------+
| sum(position="CEO") |
+---------------------+
|                   4 |
|                   1 |
+---------------------+

3 count

• 功能
  數據數量統計.

3.1 數據形式

• 不存在NULL:

圖2 表數據不存在NULL

• 存在NULL:
  

圖3 表數據存在NULL

3.2 count功能分類

序號	表達式	描述
1	count(*)	統計所有行,包括NULL
2	count(column_name)	統計列不爲NULL的行數
3	count(expression)	只要非NULL即+1,與條件表達式(expression)無關

3.2.1 count(*)

• 查詢語句:

select count(*) from userinfos group by sex;

• 查詢結果:

+----------+
| count(*) |
+----------+
|       11 |
|        5 |
+----------+

3.2.2 count(column_name)

position不存在null值:

• 查詢語句:

select count(position) from userinfos group by sex;

• 查詢結果:

+-----------------+
| count(position) |
+-----------------+
|              11 |
|               5 |
+-----------------+

position存在null值:

• 查詢語句:

select count(position) from userinfos group by sex;

• 查詢結果:

+-----------------+
| count(position) |
+-----------------+
|              10 |
|               5 |
+-----------------+

3.2.3 count(expression)

position不存在null值:

• 查詢語句:

select sex, count(position="CEO") from userinfos group by sex;

• 查詢結果:

+--------+-----------------------+
| sex    | count(position="CEO") |
+--------+-----------------------+
| female |                    11 |
| male   |                     5 |
+--------+-----------------------+

position存在null值:

• 查詢語句:

select sex, count(position="CEO") from userinfos group by sex;

• 查詢結果:

+--------+-----------------------+
| sex    | count(position="CEO") |
+--------+-----------------------+
| female |                    10 |
| male   |                     5 |
+--------+-----------------------+

4 found_rows

• 功能
  獲取分組數據的行數.

4.1 一次查詢

• 查詢語句:

select found_rows() from userinfos group by sex;

• 查詢結果:

+--------------+
| found_rows() |
+--------------+
|            2 |
|            2 |
+--------------+

返回一個列表數據,兩行,每行數據爲分組數據的行數,讀取一行數據即可,若使用MyBatis,則統計列表的長度,因爲MyBatis中返回的數據爲1,1.

4.2 兩次查詢

• 查詢語句:

select sql_calc_found_rows sex from userinfos group by sex;

• 查詢結果:

+--------+
| sex    |
+--------+
| female |
| male   |
+--------+

查詢語句:

select found_rows();

查詢結果:

+--------------+
| found_rows() |
+--------------+
|            2 |
+--------------+

5 group by

• 功能
  數據分組,細分兩個分類:單個分組字段,;多個分組字段,數據分組.

5.1 單字段分組

選擇(select)的對象,只能是分組的字段或進行統計的字段信息(如sum,count)

5.1.0 只獲取分組字段數據

• 查詢語句:

select sex from userinfos group by sex;

• 分組結果:

+--------+
| sex    |
+--------+
| female |
| male   |
+--------+

將sex字段進行分析,相同的內容進行合併,放在同一組中,返回一條數據.

5.1.2 獲取分組字段數據和統計信息

• 查詢語句:

select sex, count(*) from userinfos group by sex;

• 查詢結果:

+--------+----------+
| sex    | count(*) |
+--------+----------+
| female |       11 |
| male   |        5 |
+--------+----------+

對sex字段分析,並統計每組的所有數據數量,即female組中共有11個數據,male共有5個數據.

5.1.3 獲取統計信息

• 查詢語句:

select sex, count(*), sum(position="CEO") from userinfos group by sex;

• 查詢結果:

+--------+----------+---------------------+
| sex    | count(*) | sum(position="CEO") |
+--------+----------+---------------------+
| female |       11 |                   5 |
| male   |        5 |                   1 |
+--------+----------+---------------------+

使用sum獲取某個字段在分組中的數據,如position="CEO"的數據,female共有5個,male共有1個

5.1.4 無效語句

選擇(select)分組以外的字段,如(position),無法獲取數據.

• 查詢語句:

• 查詢結果:

ERROR 1055 (42000): Expression #2 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'data_repository.userinfos.position' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by

選擇的列表(list[sex,position])不在分組語句中(GROUP BY)並且包含了非聚合的列position,該列不在分組中,無法獲取查詢結果,因此單個字段分組時,選擇的字段必須是分組的字段.

5.2 多字段分組

• 查詢語句:

mysql> select sex from userinfos group by sex, position;

• 查詢結果:

+--------+
| sex    |
+--------+
| female |
| female |
| female |
| female |
| male   |
| male   |
| male   |
| male   |
+--------+

對sex和position字段進行過濾,先將sex字段分組,然後將position字段分析,合併查詢結果,選擇的對象爲分組對象的子集.

• 查詢語句:

select sex, position from userinfos group by sex, position;

• 查詢結果:

+--------+----------+
| sex    | position |
+--------+----------+
| female | CEO      |
| female | CFO      |
| female | COO      |
| female | CTO      |
| male   | CEO      |
| male   | CFO      |
| male   | COO      |
| male   | CTO      |
+--------+----------+

先依據sex分組,後按照position分組,相同的合併.

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