SELECT column1, column2, ... FROM table1 INNER JOIN table2 ON table1.column_name = table2.column_name;中的column1是什么意思
时间: 2023-12-16 14:04:18 浏览: 22
在SQL语句中,"SELECT"关键字用于指定要从表中检索哪些列数据,而"column1"则是指要检索的第一个列名。在进行多表查询时,可以在"FROM"关键字后面使用"INNER JOIN"来连接两个或多个表,并指定它们之间的关联条件。因此,"SELECT column1, column2, ..."语句将返回一个结果集,其中包含指定的列和相应的行数据,这些行数据来自于被连接的表。
相关问题
SELECT s.student_num, s.student_name, '计算机学院' AS college_name, c.level, c.class_name FROM student_info s JOIN classes c ON s.class_id = c.id JOIN volunteer_time_table_item t ON s.student_num = t.student_num INNER JOIN student_info s ON c.id = s.college_id INNER JOIN volunteer_time_table_item t ON s.student_num = t.student_num INNER JOIN volunteer_time_table vtt ON vtt.id = t.volunteer_time_table_id WHERE c.college_id = 1 AND s.level = 2021 AND vtt.volunteer_status = 5 AND vtt.deleted = 0 AND t.deleted = 0 AND s.deleted = 0 AND c.deleted = 0 ORDER BY t.volunteer_time_value DESC LIMIT 5;哪里有问题
这个 SQL 查询语句有以下问题:
1. 语法错误:在SELECT语句中,没有为 '计算机学院' 添加引号,应该在前后添加单引号。
2. 重复声明表别名:在JOIN语句中,重复声明了表别名 s 和 t,应该将它们改成不同的别名。
3. 错误的表连接方式:在JOIN语句中,没有为 student_info 表声明别名,应该将其改为 s 或其他合适的别名。此外,使用 INNER JOIN 连接 volunteer_time_table_item 表和 student_info 表时,应该使用 ON 关键字指定连接条件。
4. SELECT语句中重复声明列名:在SELECT语句中,声明了 student_num, student_name, level, class_name 列名,但是其中的 level 和 class_name 列名已经在 JOIN 语句中声明过了,应该删除重复的声明。
5. 语法错误:在 ORDER BY 语句中,应该为 t.volunteer_time_value 添加列名,否则会报 Unknown column 'volunteer_time_value' in 'order clause' 的错误。
修改后的 SQL 查询语句如下:
SELECT s.student_num, s.student_name, '计算机学院' AS college_name, c.level, c.class_name
FROM student_info s
JOIN classes c ON s.class_id = c.id
JOIN volunteer_time_table_item t ON s.student_num = t.student_num
INNER JOIN student_info si ON c.id = si.college_id
INNER JOIN volunteer_time_table_item ti ON si.student_num = ti.student_num
INNER JOIN volunteer_time_table vtt ON vtt.id = ti.volunteer_time_table_id
WHERE c.college_id = 1
AND s.level = 2021
AND vtt.volunteer_status = 5
AND vtt.deleted = 0
AND ti.deleted = 0
AND si.deleted = 0
AND c.deleted = 0
ORDER BY ti.volunteer_time_value DESC LIMIT 5;
begin for r in (select a.TABLE_NAME,a.COLUMN_NAME,a.DATA_TYPE,a.DATA_LENGTH,b.comments from user_tab_columns a INNER JOIN user_col_comments b on a.table_name=b.table_name and a.column_name=b.column_name where data_type='NUMBER' and data_precision='20' and data_scale='2' and a.TABLE_NAME not IN('M_TOP_TEN_CREDIT_CUST','A_G15_ALL_RELATION_TXN','A_G15_TOP_TEN_CREDIT_CUST','V_GLF_CUST_INFO_NOREPEAT','A_INT_FEE','A_NONCREDIT_TXN_ALL') AND B.comments NOT IN ('12个月同类交易笔数','关联方期初持有本行的股数','关联方期末持有本行的股数','期初持有银行股数(股)','期末持有银行股数(股)指标值')) LOOP --增加临时新字段 execute immediate 'ALTER table "'|| r.TABLE_NAME ||'" ADD "'|| r.COLUMN_NAME ||'1" NUMBER(18,2)'; --复制内容 execute immediate 'Update "'|| r.TABLE_NAME ||'" Set "'|| r.COLUMN_NAME ||'1" = "'|| r.COLUMN_NAME ||'"'; --清空原有字段值 execute immediate 'Update "'|| r.TABLE_NAME ||'" Set "'|| r.COLUMN_NAME ||'" = null'; --修改原有的字段长度 execute immediate 'ALTER table "'|| r.TABLE_NAME ||'" MODIFY "'|| r.COLUMN_NAME ||'" NUMBER(18,2)'; --把值迁回原有字段 execute immediate 'Update "'|| r.TABLE_NAME ||'" Set "'|| r.COLUMN_NAME ||'" = "'|| r.COLUMN_NAME ||'1"'; --删除新增字段 execute immediate 'ALTER table "'|| r.TABLE_NAME ||'" drop column "'|| r.COLUMN_NAME ||'1"'; end loop; end ;改写成MySQL
在 MySQL 中,可以按照以下方式重写该代码:
```
BEGIN
DECLARE done INT DEFAULT 0;
DECLARE tableName VARCHAR(100);
DECLARE columnName VARCHAR(100);
DECLARE dataType VARCHAR(100);
DECLARE dataLength INT;
DECLARE comments VARCHAR(100);
DECLARE sql_stmt VARCHAR(1000);
DECLARE cur CURSOR FOR SELECT TABLE_NAME, COLUMN_NAME, DATA_TYPE, CHARACTER_MAXIMUM_LENGTH, COLUMN_COMMENT FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_SCHEMA = 'your_database_name' AND DATA_TYPE = 'decimal' AND NUMERIC_PRECISION = 20 AND NUMERIC_SCALE = 2 AND TABLE_NAME NOT IN ('M_TOP_TEN_CREDIT_CUST','A_G15_ALL_RELATION_TXN','A_G15_TOP_TEN_CREDIT_CUST','V_GLF_CUST_INFO_NOREPEAT','A_INT_FEE','A_NONCREDIT_TXN_ALL') AND COLUMN_COMMENT NOT IN ('12个月同类交易笔数','关联方期初持有本行的股数','关联方期末持有本行的股数','期初持有银行股数(股)','期末持有银行股数(股)指标值');
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = 1;
OPEN cur;
read_loop: LOOP
FETCH cur INTO tableName, columnName, dataType, dataLength, comments;
IF done THEN
LEAVE read_loop;
END IF;
SET sql_stmt = CONCAT('ALTER TABLE `', tableName, '` MODIFY COLUMN `', columnName, '` DECIMAL(18,2)');
PREPARE stmt FROM sql_stmt;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
SET sql_stmt = CONCAT('ALTER TABLE `', tableName, '` ADD COLUMN `', columnName, '1` DECIMAL(18,2)');
PREPARE stmt FROM sql_stmt;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
SET sql_stmt = CONCAT('UPDATE `', tableName, '` SET `', columnName, '1` = `', columnName, '`');
PREPARE stmt FROM sql_stmt;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
SET sql_stmt = CONCAT('UPDATE `', tableName, '` SET `', columnName, '` = NULL');
PREPARE stmt FROM sql_stmt;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
SET sql_stmt = CONCAT('UPDATE `', tableName, '` SET `', columnName, '` = `', columnName, '1`');
PREPARE stmt FROM sql_stmt;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
SET sql_stmt = CONCAT('ALTER TABLE `', tableName, '` DROP COLUMN `', columnName, '1`');
PREPARE stmt FROM sql_stmt;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
END LOOP;
CLOSE cur;
END;
```
需要注意的是,MySQL 中 DECIMAL 数据类型的定义方式与 Oracle 中的 NUMBER 数据类型不同。在 MySQL 中,DECIMAL(18,2) 表示该字段的总长度为 18 位,小数位为 2 位。此外,MySQL 中需要使用反引号来引用表名和字段名。