MySQL UPDATE - 忽略不在 CASE 语句中的记录

Question

在使用 CASE/WHEN/THEN 时如何忽略不在 case 语句中的记录？

例如，此语句将按预期更新三个匹配的记录，但会将不匹配 WHEN/THEN 子句的所有学生记录设为 NULL

UPDATE table SET student = (CASE WHEN student = '10' THEN '100412'
                                 WHEN student = '17' THEN '100295' 
                                 WHEN student = '26' THEN '100981'
                            END)
WHERE year = '2019';

如何跳过不在 CASE 语句中的记录而只更改具有匹配子句的记录？

Answer 1

对于不在case语句中跳过记录，你可以使用类似这样的东西

UPDATE table SET student = (CASE WHEN student = '10' THEN '100412'
                                 WHEN student = '17' THEN '100295' 
                                 WHEN student = '26' THEN '100981'
                            END)
WHERE year = '2019' AND student IN ('10','17','26');

Answer 2

您可以为此使用默认情况：

UPDATE table SET student = (CASE WHEN student = '10' THEN '100412'
                                 WHEN student = '17' THEN '100295' 
                                 WHEN student = '26' THEN '100981'
                                 ELSE student
                            END)
WHERE year = '2019';

否则，如果您想最小化负载，只需将所有已知的student 值添加到WHERE 子句。这只会在那些真正受更改影响的行上触发更新

Answer 3

我认为有两种解决方案：

1. 添加 where 子句

UPDATE table SET student = (CASE WHEN student = '10' THEN '100412'
                                 WHEN student = '17' THEN '100295' 
                                 WHEN student = '26' THEN '100981'
                            END)
WHERE year = '2019' AND student IN ('10','17','26');

1. 使用 else 语句，但这将扫描整个表：

UPDATE table SET student = (CASE WHEN student = '10' THEN '100412'
                                 WHEN student = '17' THEN '100295' 
                                 WHEN student = '26' THEN '100981'
                                 ELSE student
                            END)
WHERE year = '2019';

Answer 4

您可以使用in 或= 运算符

WHERE year = '2019' AND (student = '10' or student = '17' or student  ='26');